diff --git a/build/build_config.h b/build/build_config.h
index d8c3db6ed7..f12d9b612f 100644
--- a/build/build_config.h
+++ b/build/build_config.h
@@ -73,8 +73,8 @@
 
 // For access to standard POSIXish features, use OS_POSIX instead of a
 // more specific macro.
-#if defined(OS_MACOSX) || defined(OS_LINUX) || defined(OS_FREEBSD) ||     \
-    defined(OS_OPENBSD) || defined(OS_SOLARIS) || defined(OS_ANDROID) ||  \
+#if defined(OS_MACOSX) || defined(OS_LINUX) || defined(OS_FREEBSD) ||    \
+    defined(OS_OPENBSD) || defined(OS_SOLARIS) || defined(OS_ANDROID) || \
     defined(OS_NACL) || defined(OS_QNX)
 #define OS_POSIX 1
 #endif
@@ -113,7 +113,7 @@
 #define ARCH_CPU_ARMEL 1
 #define ARCH_CPU_32_BITS 1
 #define ARCH_CPU_LITTLE_ENDIAN 1
-#elif defined(__aarch64__)
+#elif defined(__aarch64__) || defined(_M_ARM64)
 #define ARCH_CPU_ARM_FAMILY 1
 #define ARCH_CPU_ARM64 1
 #define ARCH_CPU_64_BITS 1
@@ -140,12 +140,10 @@
 // Type detection for wchar_t.
 #if defined(OS_WIN)
 #define WCHAR_T_IS_UTF16
-#elif defined(OS_POSIX) && defined(COMPILER_GCC) && \
-    defined(__WCHAR_MAX__) && \
+#elif defined(OS_POSIX) && defined(COMPILER_GCC) && defined(__WCHAR_MAX__) && \
     (__WCHAR_MAX__ == 0x7fffffff || __WCHAR_MAX__ == 0xffffffff)
 #define WCHAR_T_IS_UTF32
-#elif defined(OS_POSIX) && defined(COMPILER_GCC) && \
-    defined(__WCHAR_MAX__) && \
+#elif defined(OS_POSIX) && defined(COMPILER_GCC) && defined(__WCHAR_MAX__) && \
     (__WCHAR_MAX__ == 0x7fff || __WCHAR_MAX__ == 0xffff)
 // On Posix, we'll detect short wchar_t, but projects aren't guaranteed to
 // compile in this mode (in particular, Chrome doesn't). This is intended for
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index ce3d6eade2..3a316b36a2 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -494,13 +494,12 @@ if (custom_toolchain != "") {
   set_default_toolchain("//build/toolchain/custom")
 } else if (is_win) {
   if (is_clang) {
-    host_toolchain = "//build/toolchain/win:clang_$current_cpu"
+    host_toolchain = "//build/toolchain/win:clang_$host_cpu"
+    set_default_toolchain("//build/toolchain/win:clang_$current_cpu")
   } else {
-    host_toolchain = "//build/toolchain/win:$current_cpu"
+    host_toolchain = "//build/toolchain/win:$host_cpu"
+    set_default_toolchain("//build/toolchain/win:$current_cpu")
   }
-
-  # On windows we use the same toolchain for host and target by default.
-  set_default_toolchain("$host_toolchain")
 } else if (is_android) {
   if (host_os == "linux") {
     # Use clang for the x86/64 Linux host builds.
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 4f3544f93e..d8abe68d50 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -301,6 +301,18 @@ config("compiler") {
         "_LARGEFILE64_SOURCE",
       ]
     }
+  } else if (is_win) {
+    if (is_clang) {
+      if (current_cpu == "x86") {
+        cflags += [ "-m32" ]
+      } else if (current_cpu == "x64") {
+        cflags += [ "-m64" ]
+      } else if (current_cpu == "arm64") {
+        cflags += [ "--target=arm64-windows" ]
+      } else {
+        assert(false, "unknown current_cpu " + current_cpu)
+      }
+    }
   }
 
   if (operator_new_alignment != "default" && is_clang) {
diff --git a/build/config/win/BUILD.gn b/build/config/win/BUILD.gn
index 261a1ac4de..db29dbbe2b 100644
--- a/build/config/win/BUILD.gn
+++ b/build/config/win/BUILD.gn
@@ -41,7 +41,7 @@ config("sdk_link") {
       "$visual_studio_path\VC\lib\amd64",
       "$visual_studio_path\VC\atlmfc\lib\amd64",
     ]
-  } else {
+  } else if (current_cpu == "x86") {
     ldflags = [
       "/MACHINE:X86",
       "/SAFESEH",  # Not compatible with x64 so use only for x86.
@@ -54,6 +54,15 @@ config("sdk_link") {
     if (!is_asan) {
       ldflags += [ "/largeaddressaware" ]
     }
+  } else if (current_cpu == "arm64") {
+    ldflags = [ "/MACHINE:ARM64" ]
+    lib_dirs = [
+      "$windows_sdk_path\Lib\winv6.3\um\arm64",
+      "$visual_studio_path\VC\lib\arm64",
+      "$visual_studio_path\VC\atlmfc\lib\arm64",
+    ]
+  } else {
+    assert(false, "Unsupported CPU type")
   }
 }
 
@@ -102,6 +111,9 @@ if (current_cpu == "x64") {
   # The number after the comma is the minimum required OS version.
   # 5.02 = Windows Server 2003.
   subsystem_version_suffix = ",5.02"
+} else if (current_cpu == "arm64") {
+  # Windows ARM64 requires Windows 10.
+  subsystem_version_suffix = ",10.0"
 } else {
   # 5.01 = Windows XP.
   subsystem_version_suffix = ",5.01"
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index b9f0f3946e..ecb8d9e3fe 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+default_clang_base_path = "//third_party/llvm-build/Release+Asserts"
+
 declare_args() {
   # Path to the directory containing the VC binaries for the right
   # combination of host and target architectures. Currently only the
@@ -9,6 +11,9 @@ declare_args() {
   # If vc_bin_dir is not specified on the command line (and it normally
   # isn't), we will dynamically determine the right value to use at runtime.
   vc_bin_dir = ""
+
+  # Path to the clang toolchain.
+  clang_base_path = default_clang_base_path
 }
 
 import("//build/config/win/visual_studio_version.gni")
@@ -45,12 +50,7 @@ if (use_goma) {
   goma_prefix = ""
 }
 
-# Parameters:
-#  current_cpu: current_cpu to pass as a build arg
-#  environment: File name of environment file.
-template("msvc_toolchain") {
-  env = invoker.environment
-
+if (current_toolchain == default_toolchain) {
   if (is_debug) {
     configuration = "Debug"
   } else {
@@ -61,12 +61,29 @@ template("msvc_toolchain") {
                 "copy_dlls",
                 rebase_path(root_build_dir),
                 configuration,
-                invoker.current_cpu,
+                target_cpu,
               ])
+}
 
-  cl = invoker.cl
-
+# Parameters:
+#  toolchain_args: Settings for the toolchain, including at least:
+#     current_cpu: current_cpu to pass as a build arg
+#  environment: File name of environment file.
+template("msvc_toolchain") {
   toolchain(target_name) {
+    # When invoking this toolchain not as the default one, these args will be
+    # passed to the build. They are ignored when this is the default toolchain.
+    assert(defined(invoker.toolchain_args))
+    toolchain_args = {
+      if (defined(invoker.toolchain_args)) {
+        forward_variables_from(invoker.toolchain_args, "*")
+      }
+    }
+
+    env = invoker.environment
+
+    cl = invoker.cl
+
     # Make these apply to all tools below.
     lib_switch = ""
     lib_dir_switch = "/LIBPATH:"
@@ -77,11 +94,7 @@ template("msvc_toolchain") {
       # TODO(brettw) enable this when GN support in the binary has been rolled.
       #precompiled_header_type = "msvc"
       pdbname = "{{target_out_dir}}/{{target_output_name}}_c.pdb"
-      flags = ""
-      if (is_clang && invoker.current_cpu == "x86") {
-        flags = "-m32"
-      }
-      command = "ninja -t msvc -e $env -- $cl $flags /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
+      command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
       description = "CC {{output}}"
       outputs = [
@@ -120,16 +133,25 @@ template("msvc_toolchain") {
     }
 
     tool("asm") {
-      # TODO(brettw): "/safeseh" assembler argument is hardcoded here. Extract
-      # assembler flags to a variable like cflags. crbug.com/418613
-      if (invoker.current_cpu == "x64") {
+      is_msvc_assembler = true
+      if (toolchain_args.current_cpu == "x64") {
         ml = "ml64.exe"
         x64 = "-D_ML64_X64"
+      } else if (toolchain_args.current_cpu == "arm64") {
+        is_msvc_assembler = false
+        prefix = rebase_path("$clang_base_path/bin", root_build_dir)
+        ml = "${goma_prefix}${prefix}/clang_cl.exe --target=arm64-windows"
+        x64 = ""
       } else {
         ml = "ml.exe"
         x64 = ""
       }
-      command = "$python_path gyp-win-tool asm-wrapper $env $ml $x64 {{defines}} {{include_dirs}} {{asmflags}} /safeseh /c /Fo {{output}} {{source}}"
+      command = "$python_path gyp-win-tool asm-wrapper $env $ml $x64 {{defines}} {{include_dirs}} {{asmflags}} "
+      if (is_msvc_assembler) {
+        command += "-c -o{{output}} {{source}}"
+      } else {
+        command += "/c /Fo{{output}} {{source}}"
+      }
       outputs = [
         "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.obj",
       ]
@@ -217,52 +239,55 @@ template("msvc_toolchain") {
           "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
       description = "COPY {{source}} {{output}}"
     }
+  }
+}
 
-    # When invoking this toolchain not as the default one, these args will be
-    # passed to the build. They are ignored when this is the default toolchain.
+template("win_toolchains") {
+  assert(defined(invoker.toolchain_arch))
+  toolchain_arch = invoker.toolchain_arch
+
+  msvc_toolchain(target_name) {
+    environment = "environment." + toolchain_arch
+    cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
     toolchain_args = {
-      current_cpu = invoker.current_cpu
-      if (defined(invoker.is_clang)) {
-        is_clang = invoker.is_clang
+      if (defined(invoker.toolchain_args)) {
+        forward_variables_from(invoker.toolchain_args, "*")
       }
+      current_cpu = toolchain_arch
+      is_clang = false
+    }
+  }
+  msvc_toolchain("clang_" + target_name) {
+    environment = "environment." + toolchain_arch
+    prefix = rebase_path("$clang_base_path/bin", root_build_dir)
+    cl = "${goma_prefix}$prefix/clang-cl.exe"
+    toolchain_args = {
+      if (defined(invoker.toolchain_args)) {
+        forward_variables_from(invoker.toolchain_args, "*")
+      }
+      current_cpu = toolchain_arch
+      is_clang = true
     }
   }
 }
 
-# TODO(dpranke): Declare both toolchains all of the time when we
-# get it sorted out how we want to support them both in a single build.
-# Right now only one of these can be enabled at a time because the
-# runtime libraries get copied to root_build_dir and would collide.
-if (current_cpu == "x86") {
-  msvc_toolchain("x86") {
-    environment = "environment.x86"
-    current_cpu = "x86"
-    cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
-    is_clang = false
+if (target_cpu == "x86") {
+  win_toolchains("x86") {
+    toolchain_arch = "x86"
   }
-  msvc_toolchain("clang_x86") {
-    environment = "environment.x86"
-    current_cpu = "x86"
-    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
-                         root_build_dir)
-    cl = "${goma_prefix}$prefix/clang-cl.exe"
-    is_clang = true
+}
+
+if (target_cpu == "x64") {
+  win_toolchains("x64") {
+    toolchain_arch = "x64"
   }
 }
 
-if (current_cpu == "x64") {
-  msvc_toolchain("x64") {
-    environment = "environment.x64"
-    current_cpu = "x64"
-    cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
-    is_clang = false
+if (target_cpu == "arm64") {
+  win_toolchains("arm64") {
+    toolchain_arch = "arm64"
   }
-  msvc_toolchain("clang_x64") {
-    environment = "environment.x64"
-    current_cpu = "x64"
-    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
-                         root_build_dir)
-    cl = "${goma_prefix}$prefix/clang-cl.exe"
-    is_clang = true
+  win_toolchains(host_cpu) {
+    toolchain_arch = host_cpu
   }
 }
diff --git a/build/toolchain/win/midl.gni b/build/toolchain/win/midl.gni
index 3e7fbecca5..4110cd1781 100644
--- a/build/toolchain/win/midl.gni
+++ b/build/toolchain/win/midl.gni
@@ -63,6 +63,9 @@ template("midl") {
     } else if (current_cpu == "x64") {
       win_tool_arch = "environment.x64"
       idl_target_platform = "x64"
+    } else if (current_cpu == "arm64") {
+      win_tool_arch = "environment.arm64"
+      idl_target_platform = "arm64"
     } else {
       assert(false, "Need environment for this arch")
     }
diff --git a/build/toolchain/win/setup_toolchain.py b/build/toolchain/win/setup_toolchain.py
index 2aae35403c..8c26112b41 100644
--- a/build/toolchain/win/setup_toolchain.py
+++ b/build/toolchain/win/setup_toolchain.py
@@ -55,9 +55,8 @@ def _SetupScript(target_cpu, sdk_dir):
   """Returns a command (with arguments) to be used to set up the
   environment."""
   # Check if we are running in the SDK command line environment and use
-  # the setup script from the SDK if so. |target_cpu| should be either
-  # 'x86' or 'x64'.
-  assert target_cpu in ('x86', 'x64')
+  # the setup script from the SDK if so.
+  assert target_cpu in ('x86', 'x64', 'arm64')
   if bool(int(os.environ.get('DEPOT_TOOLS_WIN_TOOLCHAIN', 1))) and sdk_dir:
     return [os.path.normpath(os.path.join(sdk_dir, 'Bin/SetEnv.Cmd')),
             '/' + target_cpu]
@@ -109,7 +108,7 @@ def main():
 
   _CopyTool(tool_source)
 
-  cpus = ('x86', 'x64')
+  cpus = ('x86', 'x64', 'arm64')
   assert target_cpu in cpus
   vc_bin_dir = ''