diff --git a/src/FSharp.Core/FSharp.Core.fsproj b/src/FSharp.Core/FSharp.Core.fsproj
index cf267f6f914..db9075e4913 100644
--- a/src/FSharp.Core/FSharp.Core.fsproj
+++ b/src/FSharp.Core/FSharp.Core.fsproj
@@ -50,6 +50,14 @@
     <IntermediateOutputPath>$(ArtifactsDir)obj/$(MSBuildProjectName)/$(Configuration)/</IntermediateOutputPath>
   </PropertyGroup>
 
+  <ItemGroup>
+    <NuspecProperty Include="SystemCollectionsImmutablePackageVersion=$(SystemCollectionsImmutableVersion)" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="System.Collections.Immutable" Version="$(SystemCollectionsImmutableVersion)" />
+  </ItemGroup>
+
   <ItemGroup>
     <EmbeddedResource Update="FSCore.resx">
       <GenerateSource>true</GenerateSource>
@@ -138,6 +146,9 @@
     <Compile Include="array3.fs">
       <Link>Collections/array3.fs</Link>
     </Compile>
+    <Compile Include="immutablearray.fs">
+      <Link>Collections/immutablearray.fs</Link>
+    </Compile>
     <Compile Include="map.fsi">
       <Link>Collections/map.fsi</Link>
     </Compile>
diff --git a/src/FSharp.Core/FSharp.Core.nuspec b/src/FSharp.Core/FSharp.Core.nuspec
index cc7e5316ac6..34dda8708ad 100644
--- a/src/FSharp.Core/FSharp.Core.nuspec
+++ b/src/FSharp.Core/FSharp.Core.nuspec
@@ -4,8 +4,12 @@
         $CommonMetadataElements$
         <language>en-US</language>
         <dependencies>
-            <group targetFramework=".NETStandard2.0" />
-            <group targetFramework=".NETStandard2.1" />
+            <group targetFramework=".NETStandard2.0">
+                <dependency id="System.Collections.Immutable" version="$SystemCollectionsImmutablePackageVersion$" exclude="Build,Analyzers" />
+            </group>
+            <group targetFramework=".NETStandard2.1">
+                <dependency id="System.Collections.Immutable" version="$SystemCollectionsImmutablePackageVersion$" exclude="Build,Analyzers" />
+            </group>
         </dependencies>
     </metadata>
     <files>
diff --git a/src/FSharp.Core/immutablearray.fs b/src/FSharp.Core/immutablearray.fs
new file mode 100644
index 00000000000..8a4868fb95a
--- /dev/null
+++ b/src/FSharp.Core/immutablearray.fs
@@ -0,0 +1,490 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Microsoft.FSharp.Collections
+
+open System
+open System.Collections.Generic
+open System.Collections.Immutable
+
+open Microsoft.FSharp.Core
+open Microsoft.FSharp.Core.CompilerServices
+open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicOperators
+
+[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
+[<RequireQualifiedAccess>]
+module ImmutableArray =
+
+    [<CompiledName("Length")>]
+    let inline length (array: ImmutableArray<'T>) : int =
+        array.Length
+
+    [<CompiledName("Singleton")>]
+    let inline singleton (item: 'T) : ImmutableArray<_> =
+        ImmutableArray.Create(item)
+
+    [<CompiledName("Initialize")>]
+    let inline init n ([<InlineIfLambda>] f: int -> 'T) : ImmutableArray<_> =
+        match n with
+        | 0 -> ImmutableArray.Empty
+        | 1 -> ImmutableArray.Create(f 0)
+        | n ->
+            if n < 0 then
+                invalidArg "n" "Below zero."
+
+            let builder = ImmutableArray.CreateBuilder(n)
+            for i = 0 to n - 1 do
+                builder.Add(f i)
+            builder.MoveToImmutable()
+
+    [<CompiledName("ZeroCreate")>]
+    let inline zeroCreate (count: int) : ImmutableArray<'T> =
+        if count < 0 then
+            invalidArgInputMustBeNonNegative (nameof(count)) count
+
+        init count (fun _ -> LanguagePrimitives.GenericZero)
+
+    [<CompiledName("Create")>]
+    let inline create (count: int) (value: 'T) : ImmutableArray<'T> =
+        if count < 0 then
+            invalidArgInputMustBeNonNegative (nameof(count)) count
+
+        init count (fun _ -> value)
+
+    [<CompiledName("IsEmpty")>]
+    let inline isEmpty (arr: ImmutableArray<_>) = arr.IsEmpty
+
+    [<CompiledName("Empty")>]
+    [<GeneralizableValue>]
+    let empty<'T> = ImmutableArray<'T>.Empty
+
+    let inline concatImmutableArrays (arrs: ImmutableArray<ImmutableArray<'T>>) : ImmutableArray<'T> =
+        match arrs.Length with
+        | 0 -> ImmutableArray.Empty
+        | 1 -> arrs[0]
+        | 2 -> arrs[0].AddRange(arrs[1])
+        | n ->
+            let mutable acc = 0
+
+            for h in arrs do
+                acc <- acc + h.Length
+
+            let builder = ImmutableArray.CreateBuilder(acc)
+
+            for i = 0 to n - 1 do
+                builder.AddRange(arrs[i])
+
+            builder.MoveToImmutable()
+
+    [<CompiledName("Concat")>]
+    let inline concat (arrs: IEnumerable<ImmutableArray<'T>>) : ImmutableArray<'T> =
+        match arrs with
+        | :? ImmutableArray<ImmutableArray<'T>> as arrs -> concatImmutableArrays arrs
+        | arrs -> concatImmutableArrays (ImmutableArray.CreateRange(arrs))
+
+    [<CompiledName("Replicate")>]
+    let inline replicate (count: int) (value: 'T) : ImmutableArray<'T> =
+        create count value
+
+    [<CompiledName("Map")>]
+    let inline map ([<InlineIfLambda>] mapper: 'T -> 'U) (arr: ImmutableArray<'T>) : ImmutableArray<_> =
+        match arr.Length with
+        | 0 -> ImmutableArray.Empty
+        | 1 -> ImmutableArray.Create(mapper arr[0])
+        | _ ->
+            let builder = ImmutableArray.CreateBuilder(arr.Length)
+            for i = 0 to arr.Length - 1 do
+                builder.Add(mapper arr[i])
+            builder.MoveToImmutable()
+
+    [<CompiledName("MapIndexed")>]
+    let inline mapi ([<InlineIfLambda>] mapper: int -> 'T -> 'U) (arr: ImmutableArray<'T>) : ImmutableArray<_> =
+        match arr.Length with
+        | 0 -> ImmutableArray.Empty
+        | 1 -> ImmutableArray.Create(mapper 0 arr[0])
+        | n ->
+            let builder = ImmutableArray.CreateBuilder(n)
+            for i = 0 to arr.Length - 1 do
+                builder.Add(mapper i arr[i])
+            builder.MoveToImmutable()
+
+    [<CompiledName("Map2")>]
+    let map2 (mapper: 'T1 -> 'T2 -> 'T) (array1: ImmutableArray<'T1>) (array2: ImmutableArray<'T2>) : ImmutableArray<_> =
+        if array1.Length <> array2.Length then
+            invalidArgDifferentArrayLength (nameof(array1)) array1.Length (nameof(array2)) array2.Length
+
+        match array1.Length with
+        | 0 -> ImmutableArray.Empty
+        | 1 -> ImmutableArray.Create(mapper array1[0] array2[0])
+        | n ->
+            let builder = ImmutableArray.CreateBuilder(n)
+            for i = 0 to n - 1 do
+                builder.Add(mapper array1[i] array2[i])
+            builder.MoveToImmutable()
+
+    [<CompiledName("MapIndexed2")>]
+    let mapi2 (mapper: int -> 'T1 -> 'T2 -> 'T) (array1: ImmutableArray<'T1>) (array2: ImmutableArray<'T2>) : ImmutableArray<_> =
+        if array1.Length <> array2.Length then
+            invalidArgDifferentArrayLength (nameof(array1)) array1.Length (nameof(array2)) array2.Length
+
+        match array1.Length with
+        | 0 -> ImmutableArray.Empty
+        | 1 -> ImmutableArray.Create(mapper 0 array1[0] array2[0])
+        | n ->
+            let builder = ImmutableArray.CreateBuilder(n)
+            for i = 0 to n - 1 do
+                builder.Add(mapper i array1[i] array2[i])
+            builder.MoveToImmutable()
+
+    [<CompiledName("Collect")>]
+    let collect (mapper: 'T -> ImmutableArray<'U>) (array: ImmutableArray<'T>) : ImmutableArray<'U> =
+        let result = map mapper array
+        concatImmutableArrays result
+
+    [<CompiledName("Take")>]
+    let take count (array: ImmutableArray<'T>) : ImmutableArray<'T> =
+        if count < 0 then
+            invalidArgInputMustBeNonNegative (nameof(count)) count
+        elif count = 0 || array.Length = 0 then
+            empty
+        elif count > array.Length then
+            raise (InvalidOperationException(SR.GetString(SR.notEnoughElements)))
+        else
+            let builder = ImmutableArray.CreateBuilder(count)
+            builder.AddRange(array, count)
+            builder.MoveToImmutable();
+
+    [<CompiledName("TakeWhile")>]
+    let inline takeWhile ([<InlineIfLambda>] predicate) (array: ImmutableArray<'T>) : ImmutableArray<'T> =
+        let len = array.Length
+        if len = 0 then
+            empty
+        else
+            let mutable count = 0
+
+            let builder = ImmutableArray.CreateBuilder(len)
+
+            while count < len && predicate array[count] do
+                count <- count + 1
+                builder.Add(array[count])
+
+            builder.Capacity <- count
+            builder.Count <- count
+            builder.MoveToImmutable()
+
+    let inline countByImpl
+        (comparer: IEqualityComparer<'SafeKey>)
+        ([<InlineIfLambda>] projection: 'T -> 'SafeKey)
+        ([<InlineIfLambda>] getKey: 'SafeKey -> 'Key)
+        (array: ImmutableArray<'T>)
+        =
+
+        let length = array.Length
+
+        if length = 0 then
+            Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked 0
+        else
+
+            let dict = Dictionary comparer
+
+            // Build the groupings
+            for v in array do
+                let safeKey = projection v
+                let mutable prev = Unchecked.defaultof<_>
+
+                if dict.TryGetValue(safeKey, &prev) then
+                    dict[safeKey] <- prev + 1
+                else
+                    dict[safeKey] <- 1
+
+            let res = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked dict.Count
+            let mutable i = 0
+
+            for group in dict do
+                res[i] <- getKey group.Key, group.Value
+                i <- i + 1
+
+            res
+
+    // We avoid wrapping a StructBox, because under 64 JIT we get some "hard" tailcalls which affect performance
+    let inline countByValueType ([<InlineIfLambda>] projection: 'T -> 'Key) (array: ImmutableArray<'T>) =
+        countByImpl HashIdentity.Structural<'Key> projection id array
+
+    // Wrap a StructBox around all keys in case the key type is itself a type using null as a representation
+    let countByRefType (projection: 'T -> 'Key) (array: ImmutableArray<'T>) =
+        countByImpl
+            RuntimeHelpers.StructBox<'Key>.Comparer
+            (fun t -> RuntimeHelpers.StructBox(projection t))
+            (fun sb -> sb.Value)
+            array
+
+    [<CompiledName("CountBy")>]
+    let countBy (projection: 'T -> 'Key) (array: ImmutableArray<'T>) =
+        // We return regular array here, since doesn't make much sense to return an immutable array of tuples.
+        if typeof<'Key>.IsValueType then
+            countByValueType projection array
+        else
+            countByRefType projection array
+
+    [<CompiledName("Append")>]
+    let inline append (array1: ImmutableArray<'T1>) (array2: ImmutableArray<'T1>) : ImmutableArray<_> =
+        array1.AddRange(array2)
+
+    [<CompiledName("Iterate")>]
+    let inline iter ([<InlineIfLambda>] f) (arr: ImmutableArray<'T>) =
+        for i = 0 to arr.Length - 1 do
+            f arr[i]
+
+    [<CompiledName("IterateIndexed")>]
+    let inline iteri ([<InlineIfLambda>] f) (arr: ImmutableArray<'T>) =
+        for i = 0 to arr.Length - 1 do
+            f i arr[i]
+
+    [<CompiledName("Iterate2")>]
+    let iter2 f (array1: ImmutableArray<'T1>) (array2: ImmutableArray<'T2>) =
+        if array1.Length <> array2.Length then
+            invalidArgDifferentArrayLength (nameof(array1)) array1.Length (nameof(array2)) array2.Length
+
+        for i = 0 to array1.Length - 1 do
+            f array1[i] array2[i]
+
+    [<CompiledName("IterateIndexed2")>]
+    let iteri2  f (array1: ImmutableArray<'T1>) (array2: ImmutableArray<'T2>) =
+        if array1.Length <> array2.Length then
+            invalidArgDifferentArrayLength (nameof(array1)) array1.Length (nameof(array2)) array2.Length
+
+        for i = 0 to array1.Length - 1 do
+            f i array1[i] array2[i]
+
+
+    [<CompiledName("ForAll")>]
+    let inline forall ([<InlineIfLambda>] predicate) (arr: ImmutableArray<'T>) : bool =
+        let len = arr.Length
+        let rec loop i =
+            i >= len || (predicate arr[i] && loop (i+1))
+        loop 0
+
+    [<CompiledName("ForAll2")>]
+    let forall2 predicate (array1: ImmutableArray<'T1>) (array2: ImmutableArray<'T2>) : bool =
+        if array1.Length <> array2.Length then
+            invalidArgDifferentArrayLength (nameof(array1)) array1.Length (nameof(array2)) array2.Length
+
+        let f = OptimizedClosures.FSharpFunc<_, _, _>.Adapt(predicate)
+
+        let len1 = array1.Length
+        let rec loop i =
+            i >= len1 || (f.Invoke(array1[i], array2[i]) && loop (i+1))
+        loop 0
+
+    [<CompiledName("TryFind")>]
+    let inline tryFind ([<InlineIfLambda>] predicate) (arr: ImmutableArray<'T>) =
+        let len = arr.Length
+
+        let rec loop i =
+            if i >= len then
+                None
+            elif predicate arr[i] then
+                Some arr[i]
+            else
+                loop (i+1)
+
+        if len = 0 then
+            None
+        else
+            loop 0
+
+    [<CompiledName("TryFindIndex")>]
+    let inline tryFindIndex ([<InlineIfLambda>] predicate) (arr: ImmutableArray<'T>) =
+        let len = arr.Length
+
+        let rec go n =
+            if n >= len then
+                None
+            elif predicate arr[n] then
+                Some n
+            else go (n+1)
+
+        if len = 0 then
+            None
+        else
+            go 0
+
+    [<CompiledName("TryPick")>]
+    let inline tryPick ([<InlineIfLambda>] chooser) (arr: ImmutableArray<'T>) =
+        let len = arr.Length
+        let rec loop i =
+            if i >= len then
+                None
+            else
+                match chooser arr[i] with
+                | None -> loop(i+1)
+                | res -> res
+
+        if len = 0 then
+            None
+        else
+            loop 0
+
+    [<CompiledName("Head")>]
+    let inline head (array: ImmutableArray<'T>) : 'T =
+        if array.Length = 0 then
+            invalidArg (nameof(array)) LanguagePrimitives.ErrorStrings.InputArrayEmptyString
+        else
+            array[0]
+
+    [<CompiledName("TryHead")>]
+    let inline tryHead (array: ImmutableArray<'T>) : 'T option =
+        if array.Length = 0 then
+            None
+        else
+            Some array[0]
+
+    [<CompiledName("Tail")>]
+    let tail (array: ImmutableArray<'T>) : ImmutableArray<'T> =
+        // Since we target ns2.0 & ns2.1, we cannot use Slice method here.
+        let len = array.Length
+
+        if len = 0 then
+            invalidArg (nameof(array)) (SR.GetString(SR.notEnoughElements))
+        elif len = 1 then
+            singleton array[0]
+        else
+            let builder = ImmutableArray.CreateBuilder(len - 1)
+
+            for i = 1 to len - 1 do
+                builder.Add(array[i])
+
+            builder.MoveToImmutable()
+
+    [<CompiledName("Last")>]
+    let inline last (array: ImmutableArray<'T>) : 'T =
+        if array.Length = 0 then
+            invalidArg (nameof(array)) LanguagePrimitives.ErrorStrings.InputArrayEmptyString
+
+        array[array.Length - 1]
+
+    [<CompiledName("TryLast")>]
+    let inline tryLast (array: ImmutableArray<'T>) : 'T option =
+        if array.Length = 0 then
+            None
+        else
+            Some array[array.Length - 1]
+
+    [<CompiledName("SplitAt")>]
+    let splitAt index (array: ImmutableArray<'T>) =
+
+        if index < 0 then
+            invalidArgInputMustBeNonNegative "index" index
+
+        let len = array.Length
+
+        if len < index then
+            raise <| InvalidOperationException(SR.GetString(SR.notEnoughElements))
+
+        if index = 0 then
+            empty, array
+        elif index = len then
+            array, empty
+        else
+            let builderLeft  = ImmutableArray.CreateBuilder(index)
+            let builderRight = ImmutableArray.CreateBuilder(len - index)
+
+            for i = 0 to len do
+                if i < index then
+                    builderLeft.Add(array[i])
+                else
+                    builderRight.Add(array[i])
+
+            builderLeft.MoveToImmutable(), builderRight.MoveToImmutable()
+
+    [<CompiledName("ToArray")>]
+    let inline toArray (array: ImmutableArray<'T>) : 'T array =
+        let len = array.Length
+        let res = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked len
+        for i = 0 to len - 1 do
+            res[i] <- array[i]
+        res
+
+    [<CompiledName("OfArray")>]
+    let inline ofArray (list: 'T array) : ImmutableArray<'T> =
+        ImmutableArray.CreateRange list
+
+    [<CompiledName("OfSeq")>]
+    let inline ofSeq (xs: IEnumerable<'T>) =
+        ImmutableArray.CreateRange(xs)
+
+    [<CompiledName("ToList")>]
+    let inline toList (array: ImmutableArray<'T>) : 'T list =
+        List.ofSeq array
+
+    [<CompiledName("OfList")>]
+    let inline ofList (list: 'T list) : ImmutableArray<'T> =
+        ImmutableArray.CreateRange list
+
+    [<CompiledName("Filter")>]
+    let inline filter ([<InlineIfLambda>] predicate) (arr: ImmutableArray<'T>) : ImmutableArray<'T> =
+        let len = arr.Length
+        let builder = ImmutableArray.CreateBuilder(len)
+        for i = 0 to len - 1 do
+            if predicate arr[i] then
+                builder.Add(arr[i])
+        builder.Capacity <- builder.Count
+        builder.MoveToImmutable()
+
+    [<CompiledName("Exists")>]
+    let inline exists ([<InlineIfLambda>] predicate) (arr: ImmutableArray<'T>) : bool =
+        let len = arr.Length
+        let rec loop i =
+            i < len && (predicate arr[i] || loop (i+1))
+        len > 0 && loop 0
+
+    [<CompiledName("Choose")>]
+    let inline choose ([<InlineIfLambda>] chooser: 'T -> 'U option) (arr: ImmutableArray<'T>) : ImmutableArray<'U> =
+        let len = arr.Length
+        let builder = ImmutableArray.CreateBuilder(len)
+        for i = 0 to len - 1 do
+            let result = chooser arr[i]
+            if result.IsSome then
+                builder.Add(result.Value)
+        builder.Capacity <- builder.Count
+        builder.MoveToImmutable()
+
+    [<CompiledName("Distinct")>]
+    let inline distinct (array: ImmutableArray<'T>) : ImmutableArray<'T> =
+        let temp = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked array.Length
+        let mutable i = 0
+
+        let hashSet = HashSet<'T>(HashIdentity.Structural<'T>)
+
+        for v in array do
+            if hashSet.Add(v) then
+                temp[i] <- v
+                i <- i + 1
+
+        ImmutableArray.CreateRange(temp)
+
+    [<CompiledName("DistinctBy")>]
+    let inline distinctBy projection (array: ImmutableArray<'T>) : ImmutableArray<'T> =
+        let length = array.Length
+
+        if length = 0 then
+            empty
+        else
+
+            let temp = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked array.Length
+            let mutable i = 0
+            let hashSet = HashSet<_>(HashIdentity.Structural<_>)
+
+            for v in array do
+                if hashSet.Add(projection v) then
+                    temp[i] <- v
+                    i <- i + 1
+
+            ImmutableArray.CreateRange(temp)
+
+    [<CompiledName("Fold")>]
+    let inline fold ([<InlineIfLambda>] folder) state (arr: ImmutableArray<_>) =
+        let f = OptimizedClosures.FSharpFunc<_, _, _>.Adapt(folder)
+        let mutable state = state
+        for i = 0 to arr.Length - 1 do
+            state <- f.Invoke(state, arr[i])
+        state