diff --git a/pubsublite/internal/wire/subscriber.go b/pubsublite/internal/wire/subscriber.go
index 4da86acb7d2..d75b02df5c2 100644
--- a/pubsublite/internal/wire/subscriber.go
+++ b/pubsublite/internal/wire/subscriber.go
@@ -469,6 +469,10 @@ func (as *assigningSubscriber) handleAssignment(partitions partitionSet) error {
 	// Handle removed partitions.
 	for partition, subscriber := range as.subscribers {
 		if !partitions.Contains(partition) {
+			// Ignore unacked messages from this point on to avoid conflicting with
+			// the commits of the new subscriber that will be assigned this partition.
+			subscriber.Terminate()
+
 			as.unsafeRemoveService(subscriber)
 			// Safe to delete map entry during range loop:
 			// https://golang.org/ref/spec#For_statements
diff --git a/pubsublite/internal/wire/subscriber_test.go b/pubsublite/internal/wire/subscriber_test.go
index 5ebed5c1c82..0e9633d4873 100644
--- a/pubsublite/internal/wire/subscriber_test.go
+++ b/pubsublite/internal/wire/subscriber_test.go
@@ -955,6 +955,67 @@ func TestAssigningSubscriberPermanentError(t *testing.T) {
 	}
 }
 
+func TestAssigningSubscriberIgnoreOutstandingAcks(t *testing.T) {
+	const subscription = "projects/123456/locations/us-central1-b/subscriptions/my-sub"
+	receiver := newTestMessageReceiver(t)
+	msg1 := seqMsgWithOffsetAndSize(11, 100)
+	msg2 := seqMsgWithOffsetAndSize(22, 200)
+
+	verifiers := test.NewVerifiers(t)
+
+	// Assignment stream
+	asnStream := test.NewRPCVerifier(t)
+	asnStream.Push(initAssignmentReq(subscription, fakeUUID[:]), assignmentResp([]int64{1}), nil)
+	assignmentBarrier := asnStream.PushWithBarrier(assignmentAckReq(), assignmentResp([]int64{2}), nil)
+	asnStream.Push(assignmentAckReq(), nil, nil)
+	verifiers.AddAssignmentStream(subscription, asnStream)
+
+	// Partition 1
+	subStream1 := test.NewRPCVerifier(t)
+	subStream1.Push(initSubReq(subscriptionPartition{Path: subscription, Partition: 1}), initSubResp(), nil)
+	subStream1.Push(initFlowControlReq(), msgSubResp(msg1), nil)
+	verifiers.AddSubscribeStream(subscription, 1, subStream1)
+
+	cmtStream1 := test.NewRPCVerifier(t)
+	cmtStream1.Push(initCommitReq(subscriptionPartition{Path: subscription, Partition: 1}), initCommitResp(), nil)
+	// Note: no commit expected.
+	verifiers.AddCommitStream(subscription, 1, cmtStream1)
+
+	// Partition 2
+	subStream2 := test.NewRPCVerifier(t)
+	subStream2.Push(initSubReq(subscriptionPartition{Path: subscription, Partition: 2}), initSubResp(), nil)
+	subStream2.Push(initFlowControlReq(), msgSubResp(msg2), nil)
+	verifiers.AddSubscribeStream(subscription, 2, subStream2)
+
+	cmtStream2 := test.NewRPCVerifier(t)
+	cmtStream2.Push(initCommitReq(subscriptionPartition{Path: subscription, Partition: 2}), initCommitResp(), nil)
+	cmtStream2.Push(commitReq(23), commitResp(1), nil)
+	verifiers.AddCommitStream(subscription, 2, cmtStream2)
+
+	mockServer.OnTestStart(verifiers)
+	defer mockServer.OnTestEnd()
+
+	sub := newTestAssigningSubscriber(t, receiver.onMessage, subscription)
+	if gotErr := sub.WaitStarted(); gotErr != nil {
+		t.Errorf("Start() got err: (%v)", gotErr)
+	}
+
+	// Partition assignments are initially {1}.
+	ack1 := receiver.ValidateMsg(msg1)
+
+	// Partition assignments will now be {2}.
+	assignmentBarrier.Release()
+	receiver.ValidateMsg(msg2).Ack()
+
+	// Partition 1 has already been unassigned, so this ack is discarded.
+	ack1.Ack()
+
+	sub.Stop()
+	if gotErr := sub.WaitStopped(); gotErr != nil {
+		t.Errorf("Stop() got err: (%v)", gotErr)
+	}
+}
+
 func TestNewSubscriberCreatesCorrectImpl(t *testing.T) {
 	const subscription = "projects/123456/locations/us-central1-b/subscriptions/my-sub"
 	const region = "us-central1"