diff --git a/packages/query-core/src/__tests__/queriesObserver.test.tsx b/packages/query-core/src/__tests__/queriesObserver.test.tsx
index da81daa653..b8daa2c4ab 100644
--- a/packages/query-core/src/__tests__/queriesObserver.test.tsx
+++ b/packages/query-core/src/__tests__/queriesObserver.test.tsx
@@ -394,4 +394,54 @@ describe('queriesObserver', () => {
       { status: 'success', data: 102 },
     ])
   })
+
+  test('combine with stable reference should invalidate cache when query count changes', async () => {
+    const key1 = queryKey()
+    const key2 = queryKey()
+    const queryFn1 = vi.fn().mockReturnValue(1)
+    const queryFn2 = vi.fn().mockReturnValue(2)
+
+    const combine = vi.fn((results) => results)
+
+    const observer = new QueriesObserver(
+      queryClient,
+      [{ queryKey: key1, queryFn: queryFn1 }],
+      { combine },
+    )
+
+    const results: Array<Array<QueryObserverResult>> = []
+    const unsubscribe = observer.subscribe((result) => {
+      results.push(result)
+    })
+
+    try {
+      await vi.advanceTimersByTimeAsync(0)
+
+      const initialCallCount = combine.mock.calls.length
+      const baselineResults = results.length
+
+      observer.setQueries(
+        [
+          { queryKey: key1, queryFn: queryFn1 },
+          { queryKey: key2, queryFn: queryFn2 },
+        ],
+        { combine },
+      )
+
+      await vi.advanceTimersByTimeAsync(0)
+
+      expect(combine.mock.calls.length).toBeGreaterThan(initialCallCount)
+
+      expect(
+        combine.mock.calls.some(
+          (call) => Array.isArray(call[0]) && call[0].length === 2,
+        ),
+      ).toBe(true)
+
+      expect(results.length).toBeGreaterThan(baselineResults)
+      expect(results[results.length - 1]).toHaveLength(2)
+    } finally {
+      unsubscribe()
+    }
+  })
 })
diff --git a/packages/query-core/src/queriesObserver.ts b/packages/query-core/src/queriesObserver.ts
index 6458f208da..186067b02e 100644
--- a/packages/query-core/src/queriesObserver.ts
+++ b/packages/query-core/src/queriesObserver.ts
@@ -43,6 +43,8 @@ export class QueriesObserver<
   #combinedResult?: TCombinedResult
   #lastCombine?: CombineFn<TCombinedResult>
   #lastResult?: Array<QueryObserverResult>
+  // Tracks the last input passed to #combineResult to detect query count changes in optimistic updates
+  #lastInput?: Array<QueryObserverResult>
   #observerMatches: Array<QueryObserverMatch> = []
 
   constructor(
@@ -216,10 +218,13 @@ export class QueriesObserver<
       if (
         !this.#combinedResult ||
         this.#result !== this.#lastResult ||
+        // Compare input.length to handle optimistic updates where input differs from this.#result
+        input.length !== this.#lastInput?.length ||
         combine !== this.#lastCombine
       ) {
         this.#lastCombine = combine
         this.#lastResult = this.#result
+        this.#lastInput = input
         this.#combinedResult = replaceEqualDeep(
           this.#combinedResult,
           combine(input),
diff --git a/packages/react-query/src/__tests__/useQueries-combine.test.tsx b/packages/react-query/src/__tests__/useQueries-combine.test.tsx
new file mode 100644
index 0000000000..f18b78ce62
--- /dev/null
+++ b/packages/react-query/src/__tests__/useQueries-combine.test.tsx
@@ -0,0 +1,319 @@
+import { describe, expect, test } from 'vitest'
+import { renderHook, waitFor } from '@testing-library/react'
+import React from 'react'
+import {
+  QueryClient,
+  QueryClientProvider,
+  useQueries,
+} from '@tanstack/react-query'
+
+describe('useQueries combine memoization in React', () => {
+  test('stable reference combine should update immediately when queries change', () => {
+    const queryClient = new QueryClient({
+      defaultOptions: {
+        queries: {
+          retry: false,
+        },
+      },
+    })
+
+    const wrapper = ({ children }: { children: React.ReactNode }) => (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    )
+
+    const stableCombine = (results: any) => results
+
+    const { result, rerender } = renderHook(
+      ({ n }: { n: number }) => {
+        const queries = useQueries({
+          queries: [...Array(n).keys()].map((i) => ({
+            queryKey: ['stable', i],
+            queryFn: () => Promise.resolve(i + 100),
+          })),
+          combine: stableCombine,
+        })
+
+        return queries
+      },
+      {
+        wrapper,
+        initialProps: { n: 0 },
+      },
+    )
+
+    expect(result.current.length).toBe(0)
+
+    rerender({ n: 1 })
+    expect(result.current.length).toBe(1)
+
+    rerender({ n: 2 })
+    expect(result.current.length).toBe(2)
+  })
+
+  test('inline combine should update immediately when queries change', () => {
+    const queryClient = new QueryClient({
+      defaultOptions: {
+        queries: {
+          retry: false,
+        },
+      },
+    })
+
+    const wrapper = ({ children }: { children: React.ReactNode }) => (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    )
+
+    const { result, rerender } = renderHook(
+      ({ n }: { n: number }) => {
+        const queries = useQueries({
+          queries: [...Array(n).keys()].map((i) => ({
+            queryKey: ['inline', i],
+            queryFn: () => Promise.resolve(i + 100),
+          })),
+          combine: (results) => results,
+        })
+
+        return queries
+      },
+      {
+        wrapper,
+        initialProps: { n: 0 },
+      },
+    )
+
+    expect(result.current.length).toBe(0)
+
+    rerender({ n: 1 })
+    expect(result.current.length).toBe(1)
+
+    rerender({ n: 2 })
+    expect(result.current.length).toBe(2)
+  })
+})
+
+describe('useQueries combine memoization edge cases', () => {
+  test('should handle dynamic query array changes correctly', () => {
+    const queryClient = new QueryClient({
+      defaultOptions: {
+        queries: {
+          retry: false,
+        },
+      },
+    })
+
+    const wrapper = ({ children }: { children: React.ReactNode }) => (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    )
+
+    const stableCombine = (results: any) => results
+
+    const { result, rerender } = renderHook(
+      ({ ids }: { ids: Array<number> }) => {
+        const queries = useQueries({
+          queries: ids.map((id) => ({
+            queryKey: ['test', id],
+            queryFn: () => Promise.resolve(id),
+          })),
+          combine: stableCombine,
+        })
+        return queries
+      },
+      {
+        wrapper,
+        initialProps: { ids: [] } as { ids: Array<number> },
+      },
+    )
+
+    expect(result.current.length).toBe(0)
+
+    rerender({ ids: [1, 2, 3] })
+    expect(result.current.length).toBe(3)
+
+    rerender({ ids: [2, 3] })
+    expect(result.current.length).toBe(2)
+
+    rerender({ ids: [2, 3, 4, 5] })
+    expect(result.current.length).toBe(4)
+
+    rerender({ ids: [] })
+    expect(result.current.length).toBe(0)
+  })
+
+  test('should handle combine function that transforms data', () => {
+    const queryClient = new QueryClient({
+      defaultOptions: {
+        queries: {
+          retry: false,
+        },
+      },
+    })
+
+    const wrapper = ({ children }: { children: React.ReactNode }) => (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    )
+
+    const transformCombine = (results: any) => ({
+      count: results.length,
+      data: results,
+    })
+
+    const { result, rerender } = renderHook(
+      ({ n }: { n: number }) => {
+        const queries = useQueries({
+          queries: [...Array(n).keys()].map((i) => ({
+            queryKey: ['transform', i],
+            queryFn: () => Promise.resolve(i),
+          })),
+          combine: transformCombine,
+        })
+        return queries
+      },
+      {
+        wrapper,
+        initialProps: { n: 0 },
+      },
+    )
+
+    expect(result.current.count).toBe(0)
+
+    rerender({ n: 2 })
+    expect(result.current.count).toBe(2)
+    expect(result.current.data.length).toBe(2)
+
+    rerender({ n: 5 })
+    expect(result.current.count).toBe(5)
+    expect(result.current.data.length).toBe(5)
+  })
+
+  test('should not break when switching between stable and inline combine', () => {
+    const queryClient = new QueryClient({
+      defaultOptions: {
+        queries: {
+          retry: false,
+        },
+      },
+    })
+
+    const wrapper = ({ children }: { children: React.ReactNode }) => (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    )
+
+    const stableCombine = (results: any) => results
+
+    const { result, rerender } = renderHook(
+      ({ useStable }: { useStable: boolean }) => {
+        const queries = useQueries({
+          queries: [
+            {
+              queryKey: ['switch', 1],
+              queryFn: () => Promise.resolve(1),
+            },
+          ],
+          combine: useStable ? stableCombine : (results) => results,
+        })
+        return queries
+      },
+      {
+        wrapper,
+        initialProps: { useStable: true },
+      },
+    )
+
+    expect(result.current.length).toBe(1)
+
+    rerender({ useStable: false })
+    expect(result.current.length).toBe(1)
+
+    rerender({ useStable: true })
+    expect(result.current.length).toBe(1)
+  })
+
+  test('should handle same length but different queries', async () => {
+    const queryClient = new QueryClient({
+      defaultOptions: {
+        queries: {
+          retry: false,
+        },
+      },
+    })
+
+    const wrapper = ({ children }: { children: React.ReactNode }) => (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    )
+
+    const stableCombine = (results: any) => results
+
+    const { result, rerender } = renderHook(
+      ({ keys }: { keys: Array<string> }) => {
+        const queries = useQueries({
+          queries: keys.map((key) => ({
+            queryKey: [key],
+            queryFn: () => Promise.resolve(key),
+          })),
+          combine: stableCombine,
+        })
+        return queries
+      },
+      {
+        wrapper,
+        initialProps: { keys: ['a', 'b'] },
+      },
+    )
+
+    expect(result.current.length).toBe(2)
+
+    rerender({ keys: ['c', 'd'] })
+    expect(result.current.length).toBe(2)
+
+    // Note: Same-length changes may use cached result for one render cycle,
+    // but data will be correct after setQueries updates this.#result
+    await waitFor(() => {
+      expect(result.current[0]?.data).toBe('c')
+      expect(result.current[1]?.data).toBe('d')
+    })
+  })
+
+  test('should handle query order changes with same length', async () => {
+    const queryClient = new QueryClient({
+      defaultOptions: {
+        queries: {
+          retry: false,
+        },
+      },
+    })
+
+    const wrapper = ({ children }: { children: React.ReactNode }) => (
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    )
+
+    const stableCombine = (results: any) => results
+
+    const { result, rerender } = renderHook(
+      ({ keys }: { keys: Array<string> }) => {
+        const queries = useQueries({
+          queries: keys.map((key) => ({
+            queryKey: [key],
+            queryFn: () => Promise.resolve(key),
+          })),
+          combine: stableCombine,
+        })
+        return queries
+      },
+      {
+        wrapper,
+        initialProps: { keys: ['x', 'y', 'z'] },
+      },
+    )
+
+    expect(result.current.length).toBe(3)
+
+    rerender({ keys: ['z', 'x', 'y'] })
+    expect(result.current.length).toBe(3)
+
+    await waitFor(() => {
+      expect(result.current[0]?.data).toBe('z')
+      expect(result.current[1]?.data).toBe('x')
+      expect(result.current[2]?.data).toBe('y')
+    })
+  })
+})