diff --git a/bson/primitive/objectid_test.go b/bson/primitive/objectid_test.go
index 36f64a2b9a..5ed296120b 100644
--- a/bson/primitive/objectid_test.go
+++ b/bson/primitive/objectid_test.go
@@ -37,6 +37,13 @@ func BenchmarkObjectIDFromHex(b *testing.B) {
 	}
 }
 
+func BenchmarkNewObjectIDFromTimestamp(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		timestamp := time.Now().Add(time.Duration(i) * time.Millisecond)
+		_ = NewObjectIDFromTimestamp(timestamp)
+	}
+}
+
 func TestFromHex_RoundTrip(t *testing.T) {
 	before := NewObjectID()
 	after, err := ObjectIDFromHex(before.Hex())
diff --git a/mongo/bulk_write.go b/mongo/bulk_write.go
index a7efd551e7..f054c1e7ac 100644
--- a/mongo/bulk_write.go
+++ b/mongo/bulk_write.go
@@ -171,7 +171,7 @@ func (bw *bulkWrite) runInsert(ctx context.Context, batch bulkWriteBatch) (opera
 		if err != nil {
 			return operation.InsertResult{}, err
 		}
-		doc, _, err = ensureID(doc, primitive.NewObjectID(), bw.collection.bsonOpts, bw.collection.registry)
+		doc, _, err = ensureID(doc, primitive.NilObjectID, bw.collection.bsonOpts, bw.collection.registry)
 		if err != nil {
 			return operation.InsertResult{}, err
 		}
diff --git a/mongo/collection.go b/mongo/collection.go
index 37214b9153..477363b57a 100644
--- a/mongo/collection.go
+++ b/mongo/collection.go
@@ -299,7 +299,7 @@ func (coll *Collection) insert(ctx context.Context, documents []interface{},
 		if err != nil {
 			return nil, err
 		}
-		bsoncoreDoc, id, err := ensureID(bsoncoreDoc, primitive.NewObjectID(), coll.bsonOpts, coll.registry)
+		bsoncoreDoc, id, err := ensureID(bsoncoreDoc, primitive.NilObjectID, coll.bsonOpts, coll.registry)
 		if err != nil {
 			return nil, err
 		}
diff --git a/mongo/mongo.go b/mongo/mongo.go
index 4fd4c1548a..d0e321f7bf 100644
--- a/mongo/mongo.go
+++ b/mongo/mongo.go
@@ -153,8 +153,11 @@ func marshal(
 }
 
 // ensureID inserts the given ObjectID as an element named "_id" at the
-// beginning of the given BSON document if there is not an "_id" already. If
-// there is already an element named "_id", the document is not modified. It
+// beginning of the given BSON document if there is not an "_id" already.
+// If the given ObjectID is primitive.NilObjectID, a new object ID will be
+// generated with time.Now().
+//
+// If there is already an element named "_id", the document is not modified. It
 // returns the resulting document and the decoded Go value of the "_id" element.
 func ensureID(
 	doc bsoncore.Document,
@@ -195,6 +198,9 @@ func ensureID(
 	const extraSpace = 17
 	doc = make(bsoncore.Document, 0, len(olddoc)+extraSpace)
 	_, doc = bsoncore.ReserveLength(doc)
+	if oid.IsZero() {
+		oid = primitive.NewObjectID()
+	}
 	doc = bsoncore.AppendObjectIDElement(doc, "_id", oid)
 
 	// Remove and re-write the BSON document length header.
diff --git a/mongo/mongo_test.go b/mongo/mongo_test.go
index 8055236a86..b17422ce1e 100644
--- a/mongo/mongo_test.go
+++ b/mongo/mongo_test.go
@@ -134,6 +134,29 @@ func TestEnsureID(t *testing.T) {
 	}
 }
 
+func TestEnsureID_NilObjectID(t *testing.T) {
+	t.Parallel()
+
+	doc := bsoncore.NewDocumentBuilder().
+		AppendString("foo", "bar").
+		Build()
+
+	got, gotIDI, err := ensureID(doc, primitive.NilObjectID, nil, nil)
+	assert.NoError(t, err)
+
+	gotID, ok := gotIDI.(primitive.ObjectID)
+
+	assert.True(t, ok)
+	assert.NotEqual(t, primitive.NilObjectID, gotID)
+
+	want := bsoncore.NewDocumentBuilder().
+		AppendObjectID("_id", gotID).
+		AppendString("foo", "bar").
+		Build()
+
+	assert.Equal(t, want, got)
+}
+
 func TestMarshalAggregatePipeline(t *testing.T) {
 	// []byte of [{{"$limit", 12345}}]
 	index, arr := bsoncore.AppendArrayStart(nil)