diff --git a/.changeset/five-waves-rule.md b/.changeset/five-waves-rule.md
new file mode 100644
index 000000000..020f9bbba
--- /dev/null
+++ b/.changeset/five-waves-rule.md
@@ -0,0 +1,5 @@
+---
+"@tanstack/db": patch
+---
+
+Fix joins using conditions with computed values (such as `concat()`)
diff --git a/packages/db/src/query/compiler/joins.ts b/packages/db/src/query/compiler/joins.ts
index 42687e446..50326339e 100644
--- a/packages/db/src/query/compiler/joins.ts
+++ b/packages/db/src/query/compiler/joins.ts
@@ -204,7 +204,12 @@ function processJoin(
       lazyFrom.type === `queryRef` &&
       (lazyFrom.query.limit || lazyFrom.query.offset)
 
-    if (!limitedSubquery) {
+    // If join expressions contain computed values (like concat functions)
+    // we don't optimize the join because we don't have an index over the computed values
+    const hasComputedJoinExpr =
+      mainExpr.type === `func` || joinedExpr.type === `func`
+
+    if (!limitedSubquery && !hasComputedJoinExpr) {
       // This join can be optimized by having the active collection
       // dynamically load keys into the lazy collection
       // based on the value of the joinKey and by looking up
diff --git a/packages/db/tests/query/join.test.ts b/packages/db/tests/query/join.test.ts
index 9b2d3ef24..dc0eca84f 100644
--- a/packages/db/tests/query/join.test.ts
+++ b/packages/db/tests/query/join.test.ts
@@ -1,5 +1,5 @@
 import { beforeEach, describe, expect, test } from "vitest"
-import { createLiveQueryCollection, eq } from "../../src/query/index.js"
+import { concat, createLiveQueryCollection, eq } from "../../src/query/index.js"
 import { createCollection } from "../../src/collection/index.js"
 import { mockSyncCollectionOptions } from "../utils.js"
 
@@ -948,6 +948,30 @@ function createJoinTests(autoIndex: `off` | `eager`): void {
         const alice = results.find((r) => r.user_name === `Alice`)
         expect(alice?.department_name).toBe(`Engineering`)
       })
+
+      test(`should handle joins with computed expressions`, () => {
+        const joinQuery = createLiveQueryCollection({
+          startSync: true,
+          query: (q) =>
+            q
+              .from({ user: usersCollection })
+              .join(
+                { dept: departmentsCollection },
+                ({ user, dept }) =>
+                  eq(
+                    concat(`dept`, user.department_id),
+                    concat(`dept`, dept.id)
+                  ),
+                `inner`
+              )
+              .select(({ user, dept }) => ({
+                user_name: user.name,
+                department_name: dept.name,
+              })),
+        })
+
+        expect(joinQuery.size).toBe(3)
+      })
     })
 
     test(`should handle chained joins with incremental updates`, () => {