diff --git a/sql/core/src/main/scala/org/apache/spark/sql/UDFRegistration.scala b/sql/core/src/main/scala/org/apache/spark/sql/UDFRegistration.scala
index a4ff09596ad8e..0f08e10c00d22 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/UDFRegistration.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/UDFRegistration.scala
@@ -73,7 +73,11 @@ class UDFRegistration private[sql] (functionRegistry: FunctionRegistry) extends
    * @return the registered UDAF.
    *
    * @since 1.5.0
+   * @deprecated this method and the use of UserDefinedAggregateFunction are deprecated.
+   * Aggregator[IN, BUF, OUT] should now be registered as a UDF via the functions.udaf(agg) method.
    */
+  @deprecated("Aggregator[IN, BUF, OUT] should now be registered as a UDF" +
+    " via the functions.udaf(agg) method.", "3.0.0")
   def register(name: String, udaf: UserDefinedAggregateFunction): UserDefinedAggregateFunction = {
     def builder(children: Seq[Expression]) = ScalaUDAF(children, udaf)
     functionRegistry.createOrReplaceTempFunction(name, builder)
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/expressions/udaf.scala b/sql/core/src/main/scala/org/apache/spark/sql/expressions/udaf.scala
index 4e8cb3a6ddd66..8407b1419af62 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/expressions/udaf.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/expressions/udaf.scala
@@ -27,8 +27,12 @@ import org.apache.spark.sql.types._
  * The base class for implementing user-defined aggregate functions (UDAF).
  *
  * @since 1.5.0
+ * @deprecated UserDefinedAggregateFunction is deprecated.
+ * Aggregator[IN, BUF, OUT] should now be registered as a UDF via the functions.udaf(agg) method.
  */
 @Stable
+@deprecated("Aggregator[IN, BUF, OUT] should now be registered as a UDF" +
+  " via the functions.udaf(agg) method.", "3.0.0")
 abstract class UserDefinedAggregateFunction extends Serializable {
 
   /**