diff --git a/.changeset/brave-coats-return.md b/.changeset/brave-coats-return.md
new file mode 100644
index 000000000..02bf1c0ea
--- /dev/null
+++ b/.changeset/brave-coats-return.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/db': minor
+---
+
+Added writeUpsert util to localstorage collection
diff --git a/packages/db/src/local-storage.ts b/packages/db/src/local-storage.ts
index 3060b7ec6..9591af6fb 100644
--- a/packages/db/src/local-storage.ts
+++ b/packages/db/src/local-storage.ts
@@ -120,6 +120,23 @@ export interface LocalStorageCollectionUtils extends UtilsRecord {
   acceptMutations: (transaction: {
     mutations: Array<PendingMutation<Record<string, unknown>>>
   }) => void
+  /**
+   * Upsert one or more items directly into the synced data store without triggering optimistic updates.
+   * If an item with the same key exists, it will be updated; otherwise, it will be inserted.
+   *
+   * @param data - The item or array of items to upsert
+   * @example
+   * // Upsert a single item
+   * localSettings.utils.writeUpsert({ id: 'theme', value: 'dark' })
+   *
+   * @example
+   * // Upsert multiple items
+   * localSettings.utils.writeUpsert([
+   *   { id: 'theme', value: 'dark' },
+   *   { id: 'language', value: 'en' }
+   * ])
+   */
+  writeUpsert: <T extends object>(data: T | Array<T>) => void
 }
 
 /**
@@ -528,6 +545,57 @@ export function localStorageCollectionOptions(
     return handlerResult
   }
 
+  /**
+   * Upsert utility function that directly persists to localStorage
+   * Inserts if item doesn't exist, updates if it does
+   */
+  const writeUpsert = <T extends object>(data: T | Array<T>): void => {
+    const items = Array.isArray(data) ? data : [data]
+
+    // Validate that all values can be JSON serialized
+    items.forEach((item) => {
+      validateJsonSerializable(parser, item, `upsert`)
+    })
+
+    // Prepare change messages for sync confirmation
+    const mutations: Array<PendingMutation<T>> = []
+
+    // Process each item: check if exists, then insert or update
+    items.forEach((item) => {
+      const key = config.getKey(item as any)
+      const existsInStorage = lastKnownData.has(key)
+      const operationType = existsInStorage ? `update` : `insert`
+
+      // For updates, merge with existing data; for inserts, use item as-is
+      const finalData = existsInStorage
+        ? { ...lastKnownData.get(key)!.data, ...item }
+        : item
+
+      const storedItem: StoredItem<T> = {
+        versionKey: generateUuid(),
+        data: finalData as T,
+      }
+      lastKnownData.set(key, storedItem)
+
+      // Track mutation for sync confirmation
+      mutations.push({
+        type: operationType,
+        key,
+        modified: finalData as T,
+        original: existsInStorage
+          ? (lastKnownData.get(key)?.data as T)
+          : ({} as T),
+        collection: sync.collection,
+      } as PendingMutation<T>)
+    })
+
+    // Save to storage
+    saveToStorage(lastKnownData)
+
+    // Confirm mutations through sync interface
+    sync.confirmOperationsSync(mutations)
+  }
+
   // Extract standard Collection config properties
   const {
     storageKey: _storageKey,
@@ -617,6 +685,7 @@ export function localStorageCollectionOptions(
       clearStorage,
       getStorageSize,
       acceptMutations,
+      writeUpsert,
     },
   }
 }
diff --git a/packages/db/tests/local-storage.test.ts b/packages/db/tests/local-storage.test.ts
index 878e570ea..ee5dffaf9 100644
--- a/packages/db/tests/local-storage.test.ts
+++ b/packages/db/tests/local-storage.test.ts
@@ -2102,4 +2102,166 @@ describe(`localStorage collection`, () => {
       subscription.unsubscribe()
     })
   })
+
+  describe(`writeUpsert utility`, () => {
+    it(`should insert a single item when it doesn't exist`, () => {
+      const collection = createCollection(
+        localStorageCollectionOptions<Todo>({
+          storageKey: `todos`,
+          storage: mockStorage,
+          storageEventApi: mockStorageEventApi,
+          getKey: (todo) => todo.id,
+        }),
+      )
+
+      const subscription = collection.subscribeChanges(() => {})
+
+      const todo: Todo = {
+        id: `upsert-1`,
+        title: `Upsert Insert`,
+        completed: false,
+        createdAt: new Date(),
+      }
+
+      collection.utils.writeUpsert(todo)
+
+      expect(collection.has(`upsert-1`)).toBe(true)
+      expect(collection.get(`upsert-1`)?.title).toBe(`Upsert Insert`)
+
+      const storedData = mockStorage.getItem(`todos`)
+      expect(storedData).toBeDefined()
+      const parsed = JSON.parse(storedData!)
+      expect(parsed[`s:upsert-1`].data.title).toBe(`Upsert Insert`)
+
+      subscription.unsubscribe()
+    })
+
+    it(`should update a single item when it exists`, () => {
+      const collection = createCollection(
+        localStorageCollectionOptions<Todo>({
+          storageKey: `todos`,
+          storage: mockStorage,
+          storageEventApi: mockStorageEventApi,
+          getKey: (todo) => todo.id,
+        }),
+      )
+
+      const subscription = collection.subscribeChanges(() => {})
+
+      const initialTodo: Todo = {
+        id: `upsert-2`,
+        title: `Initial`,
+        completed: false,
+        createdAt: new Date(),
+      }
+
+      collection.utils.writeUpsert(initialTodo)
+
+      const updatedTodo: Todo = {
+        id: `upsert-2`,
+        title: `Updated`,
+        completed: true,
+        createdAt: new Date(),
+      }
+
+      collection.utils.writeUpsert(updatedTodo)
+
+      expect(collection.has(`upsert-2`)).toBe(true)
+      expect(collection.get(`upsert-2`)?.title).toBe(`Updated`)
+      expect(collection.get(`upsert-2`)?.completed).toBe(true)
+
+      const storedData = mockStorage.getItem(`todos`)
+      expect(storedData).toBeDefined()
+      const parsed = JSON.parse(storedData!)
+      expect(parsed[`s:upsert-2`].data.title).toBe(`Updated`)
+      expect(parsed[`s:upsert-2`].data.completed).toBe(true)
+
+      subscription.unsubscribe()
+    })
+
+    it(`should merge data when updating existing item`, () => {
+      const collection = createCollection(
+        localStorageCollectionOptions<Todo>({
+          storageKey: `todos`,
+          storage: mockStorage,
+          storageEventApi: mockStorageEventApi,
+          getKey: (todo) => todo.id,
+        }),
+      )
+
+      const subscription = collection.subscribeChanges(() => {})
+
+      const initialTodo: Todo = {
+        id: `upsert-3`,
+        title: `Initial Title`,
+        completed: false,
+        createdAt: new Date(`2024-01-01`),
+      }
+
+      collection.utils.writeUpsert(initialTodo)
+
+      const partialUpdate = {
+        id: `upsert-3`,
+        completed: true,
+      } as Partial<Todo>
+
+      collection.utils.writeUpsert(partialUpdate)
+
+      const item = collection.get(`upsert-3`)
+      expect(item?.title).toBe(`Initial Title`)
+      expect(item?.completed).toBe(true)
+      expect(item?.createdAt).toEqual(new Date(`2024-01-01`))
+
+      const storedData = mockStorage.getItem(`todos`)
+      expect(storedData).toBeDefined()
+      const parsed = JSON.parse(storedData!)
+      expect(parsed[`s:upsert-3`].data.title).toBe(`Initial Title`)
+      expect(parsed[`s:upsert-3`].data.completed).toBe(true)
+
+      subscription.unsubscribe()
+    })
+
+    it(`should upsert multiple items from an array`, () => {
+      const collection = createCollection(
+        localStorageCollectionOptions<Todo>({
+          storageKey: `todos`,
+          storage: mockStorage,
+          storageEventApi: mockStorageEventApi,
+          getKey: (todo) => todo.id,
+        }),
+      )
+
+      const subscription = collection.subscribeChanges(() => {})
+
+      const todos: Array<Todo> = [
+        {
+          id: `multi-1`,
+          title: `First`,
+          completed: false,
+          createdAt: new Date(),
+        },
+        {
+          id: `multi-2`,
+          title: `Second`,
+          completed: false,
+          createdAt: new Date(),
+        },
+      ]
+
+      collection.utils.writeUpsert(todos)
+
+      expect(collection.has(`multi-1`)).toBe(true)
+      expect(collection.has(`multi-2`)).toBe(true)
+      expect(collection.get(`multi-1`)?.title).toBe(`First`)
+      expect(collection.get(`multi-2`)?.title).toBe(`Second`)
+
+      const storedData = mockStorage.getItem(`todos`)
+      expect(storedData).toBeDefined()
+      const parsed = JSON.parse(storedData!)
+      expect(parsed[`s:multi-1`].data.title).toBe(`First`)
+      expect(parsed[`s:multi-2`].data.title).toBe(`Second`)
+
+      subscription.unsubscribe()
+    })
+  })
 })