diff --git a/Firestore/CHANGELOG.md b/Firestore/CHANGELOG.md
index 1074d97f600..ebd09bd36fa 100644
--- a/Firestore/CHANGELOG.md
+++ b/Firestore/CHANGELOG.md
@@ -1,3 +1,7 @@
+# Unreleased
+
+- [feature] Added support for 16 MB documents by increasing gRPC message size limit to 17MB [#16206].
+
 # 12.16.0
 - [changed] Migrates the network connectivity monitoring implementation for
   Apple platforms from the legacy SCNetworkReachability API to the modern
diff --git a/Firestore/Swift/Tests/Integration/LargeDocIntegrationTests.swift b/Firestore/Swift/Tests/Integration/LargeDocIntegrationTests.swift
new file mode 100644
index 00000000000..c1a7164e4ae
--- /dev/null
+++ b/Firestore/Swift/Tests/Integration/LargeDocIntegrationTests.swift
@@ -0,0 +1,191 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import FirebaseFirestore
+import Foundation
+import XCTest
+
+@available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
+class LargeDocIntegrationTests: FSTIntegrationTestCase {
+  override func setUpWithError() throws {
+    try super.setUpWithError()
+
+    // Skip by default to prevent slowing down CI.
+    // Add environment variable FIRESTORE_RUN_LARGE_DOC_TESTS = YES.
+    let runLargeTests = ProcessInfo.processInfo.environment["FIRESTORE_RUN_LARGE_DOC_TESTS"]
+    if runLargeTests != "YES" && runLargeTests != "true" {
+      throw XCTSkip("Skipping large doc tests. Set FIRESTORE_RUN_LARGE_DOC_TESTS=YES to run.")
+    }
+
+    // Skip tests if the backend edition is not supported or if not nightly
+    if FSTIntegrationTestCase.targetBackend() != .nightly || FSTIntegrationTestCase
+      .backendEdition() == .standard {
+      throw XCTSkip("Skipping large document tests because backend is not compatible.")
+    }
+  }
+
+  override class func tearDown() {
+    super.tearDown()
+  }
+
+  // MARK: - Helper Methods
+
+  private func generateString(sizeInBytes: Int, character: Character = "a") -> String {
+    return String(repeating: character, count: sizeInBytes)
+  }
+
+  private static var largeDocument: [String: Sendable] =
+    ["chunk": String(repeating: "a", count: Int(15.9 * 1024 * 1024))]
+
+  // MARK: - Test Cases
+
+  // NOTE: This test is currently expected to fail, because the payload
+  // size exceeds the gRPC message size limit. The Unicode character is
+  // encoded as 4 bytes, which causes the 16 MB document to be encoded
+  // as 64 MB.
+  func testReadAndCacheLargeUnicodeDocument() async throws {
+    throw XCTSkip("Skipping unicode document test.")
+
+    let colRef = collectionRef()
+    let db = colRef.firestore
+    let docRef = colRef.document("doc_15_9MB_unicode")
+    try await docRef
+      .setData(["chunk": generateString(sizeInBytes: Int(15.9 * 1024 * 1024), character: "🚀")])
+
+    let serverSnapshot = try await docRef.getDocument(source: .server)
+    XCTAssertTrue(serverSnapshot.exists)
+
+    try await db.disableNetwork()
+    defer { Task { try? await db.enableNetwork() } }
+
+    let cacheSnapshot = try await docRef.getDocument(source: .cache)
+    XCTAssertTrue(cacheSnapshot.exists)
+
+    let serverData = serverSnapshot.data() as NSDictionary?
+    let cacheData = cacheSnapshot.data() as NSDictionary?
+    XCTAssertEqual(serverData, cacheData)
+  }
+
+  func testQueryLargeDocumentsForcesLocalScan() async throws {
+    let colRef = collectionRef()
+    let db = colRef.firestore
+    let docRefA = colRef.document("doc_a")
+    let docRefB = colRef.document("doc_b")
+    try await docRefA.setData(LargeDocIntegrationTests.largeDocument)
+    try await docRefB.setData(LargeDocIntegrationTests.largeDocument)
+
+    try await docRefA.getDocument(source: .server)
+    try await docRefB.getDocument(source: .server)
+
+    try await db.disableNetwork()
+    defer { Task { try? await db.enableNetwork() } }
+
+    // Execute offline query which requires full local index scan
+    let query = colRef.order(by: FieldPath.documentID()).limit(to: 2)
+    let cacheSnapshot = try await query.getDocuments(source: .cache)
+
+    XCTAssertEqual(cacheSnapshot.documents.count, 2)
+    if let firstDoc = cacheSnapshot.documents.first {
+      XCTAssertTrue(firstDoc.data().count > 0)
+    }
+  }
+
+  func testWatchStreamInitializationAndDiff() async throws {
+    let colRef = collectionRef()
+    let db = colRef.firestore
+    let docRef = colRef.document("doc_a")
+    try await docRef.setData(LargeDocIntegrationTests.largeDocument)
+
+    let expectation = XCTestExpectation(description: "Wait for differential update payload")
+
+    let listener = docRef.addSnapshotListener { snapshot, error in
+      XCTAssertNil(error)
+      guard let snapshot = snapshot else { return }
+
+      if snapshot.exists && snapshot.data()?["differential_field"] != nil {
+        expectation.fulfill()
+      }
+    }
+    defer { listener.remove() }
+
+    try await docRef.updateData(["differential_field": "updated_value"])
+    await fulfillment(of: [expectation], timeout: 60)
+  }
+
+  func testOversizedPayloadRejection() async throws {
+    let docRef = collectionRef().document("temp_oversized_doc")
+    let targetBytes = (16 * 1024 * 1024) + 102_400
+    let largePayload = generateString(sizeInBytes: targetBytes)
+
+    do {
+      try await docRef.setData(["largeField": largePayload])
+      XCTFail("Setting a document exceeding the 16MB limit should fail.")
+    } catch {
+      let nsError = error as NSError
+      XCTAssertEqual(nsError.domain, FirestoreErrorDomain)
+      // Must map to standard InvalidArgument error, not a crash.
+      XCTAssertEqual(nsError.code, FirestoreErrorCode.invalidArgument.rawValue)
+    }
+  }
+
+  func testQueryLargeDocuments() async throws {
+    let colRef = collectionRef()
+    let db = colRef.firestore
+    let docRefA = colRef.document("doc_a")
+    let docRefB = colRef.document("doc_b")
+    try await docRefA.setData(LargeDocIntegrationTests.largeDocument)
+    try await docRefB.setData(LargeDocIntegrationTests.largeDocument)
+
+    let query = colRef.whereField(FieldPath.documentID(), in: ["doc_a", "doc_b"])
+
+    let serverSnapshot = try await query.getDocuments(source: .server)
+    XCTAssertEqual(serverSnapshot.documents.count, 2)
+
+    try await db.disableNetwork()
+    defer { Task { try? await db.enableNetwork() } }
+
+    let cacheSnapshot = try await query.getDocuments(source: .cache)
+    XCTAssertEqual(cacheSnapshot.documents.count, 2)
+
+    let serverFirstData = serverSnapshot.documents.first?.data() as NSDictionary?
+    let cacheFirstData = cacheSnapshot.documents.first?.data() as NSDictionary?
+    XCTAssertEqual(serverFirstData, cacheFirstData)
+  }
+
+  func testTransactionReadModifyWrite() async throws {
+    let colRef = collectionRef()
+    let db = colRef.firestore
+    let docRef = colRef.document("doc_a")
+    try await docRef.setData(LargeDocIntegrationTests.largeDocument)
+
+    do {
+      _ = try await db.runTransaction { transaction, errorPointer -> Any? in
+        do {
+          _ = try transaction.getDocument(docRef)
+          transaction.updateData(
+            ["transaction_timestamp": FieldValue.serverTimestamp()],
+            forDocument: docRef
+          )
+        } catch let fetchError as NSError {
+          errorPointer?.pointee = fetchError
+        }
+        return nil
+      }
+    } catch {
+      XCTFail("Transaction failed with error: \(error)")
+    }
+  }
+}
diff --git a/Firestore/core/src/remote/grpc_connection.cc b/Firestore/core/src/remote/grpc_connection.cc
index 78de37b5636..d37f3850d64 100644
--- a/Firestore/core/src/remote/grpc_connection.cc
+++ b/Firestore/core/src/remote/grpc_connection.cc
@@ -306,6 +306,12 @@ std::shared_ptr<grpc::Channel> GrpcConnection::CreateChannel() const {
   // This acts as a failsafe.)
   args.SetInt(GRPC_ARG_KEEPALIVE_TIME_MS, 30 * 1000);
 
+  // Increase the max message size to 17MB, to support 16MB documents
+  // + overhead.
+  const int GRPC_MAX_MESSAGE_SIZE = 17 * 1024 * 1024;
+  args.SetMaxReceiveMessageSize(GRPC_MAX_MESSAGE_SIZE);
+  args.SetMaxSendMessageSize(GRPC_MAX_MESSAGE_SIZE);
+
   const HostConfig* host_config = Config().find(host);
   if (!host_config) {
     std::string root_certificate = LoadGrpcRootCertificate();