diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/adaptive/OptimizeSkewedJoin.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/adaptive/OptimizeSkewedJoin.scala
index 396c9c9d6b4e5..f348b1cb86e5b 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/adaptive/OptimizeSkewedJoin.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/adaptive/OptimizeSkewedJoin.scala
@@ -250,10 +250,7 @@ case class OptimizeSkewedJoin(conf: SQLConf) extends Rule[SparkPlan] {
       }
   }
 
-  override def apply(plan: SparkPlan): SparkPlan = {
-    if (!conf.getConf(SQLConf.SKEW_JOIN_ENABLED)) {
-      return plan
-    }
+  private def tryOptimize(plan: SparkPlan): SparkPlan = {
 
     def collectShuffleStages(plan: SparkPlan): Seq[ShuffleQueryStageExec] = plan match {
       case stage: ShuffleQueryStageExec => Seq(stage)
@@ -286,6 +283,36 @@ case class OptimizeSkewedJoin(conf: SQLConf) extends Rule[SparkPlan] {
       plan
     }
   }
+
+  override def apply(plan: SparkPlan): SparkPlan = {
+    if (!conf.getConf(SQLConf.SKEW_JOIN_ENABLED)) {
+      return plan
+    }
+
+    // Try to handle skew join with union case, like
+    // Union
+    //   SMJ
+    //     Sort
+    //       Shuffle
+    //     Sort
+    //       Shuffle
+    //   SMJ
+    //     Sort
+    //       Shuffle
+    //     Sort
+    //       Shuffle
+    var containsUnion = false
+    val optimizedUnion = plan transformUp {
+      case u @ UnionExec(children) =>
+        containsUnion = true
+        u.withNewChildren(children.map(tryOptimize))
+    }
+    if (containsUnion) {
+      optimizedUnion
+    } else {
+      tryOptimize(plan)
+    }
+  }
 }
 
 private object ShuffleStage {
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/adaptive/AdaptiveQueryExecSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/adaptive/AdaptiveQueryExecSuite.scala
index 9fa97bffa8910..cf34a6a856f8d 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/adaptive/AdaptiveQueryExecSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/adaptive/AdaptiveQueryExecSuite.scala
@@ -26,7 +26,7 @@ import org.apache.spark.scheduler.{SparkListener, SparkListenerEvent, SparkListe
 import org.apache.spark.sql.{QueryTest, Row, SparkSession, Strategy}
 import org.apache.spark.sql.catalyst.optimizer.{BuildLeft, BuildRight}
 import org.apache.spark.sql.catalyst.plans.logical.{Aggregate, LogicalPlan}
-import org.apache.spark.sql.execution.{PartialReducerPartitionSpec, ReusedSubqueryExec, ShuffledRowRDD, SparkPlan}
+import org.apache.spark.sql.execution.{PartialReducerPartitionSpec, ReusedSubqueryExec, ShuffledRowRDD, SparkPlan, UnionExec}
 import org.apache.spark.sql.execution.command.DataWritingCommandExec
 import org.apache.spark.sql.execution.exchange.{BroadcastExchangeExec, Exchange, ReusedExchangeExec}
 import org.apache.spark.sql.execution.joins.{BroadcastHashJoinExec, SortMergeJoinExec}
@@ -719,6 +719,105 @@ class AdaptiveQueryExecSuite
     }
   }
 
+  test("SPARK-32129: adaptive skew join with union") {
+    withSQLConf(
+      SQLConf.ADAPTIVE_EXECUTION_ENABLED.key -> "true",
+      SQLConf.AUTO_BROADCASTJOIN_THRESHOLD.key -> "-1",
+      SQLConf.COALESCE_PARTITIONS_MIN_PARTITION_NUM.key -> "1",
+      SQLConf.SHUFFLE_PARTITIONS.key -> "100",
+      SQLConf.SKEW_JOIN_SKEWED_PARTITION_THRESHOLD.key -> "800",
+      SQLConf.ADVISORY_PARTITION_SIZE_IN_BYTES.key -> "800") {
+      withTempView("skewData1", "skewData2", "skewData3", "skewData4") {
+        spark
+          .range(0, 1000, 1, 10)
+          .select(
+            when('id < 250, 249)
+              .when('id >= 750, 1000)
+              .otherwise('id).as("key1"),
+            'id as "value1")
+          .createOrReplaceTempView("skewData1")
+        spark
+          .range(0, 1000, 1, 10)
+          .select(
+            when('id < 250, 249)
+              .otherwise('id).as("key2"),
+            'id as "value2")
+          .createOrReplaceTempView("skewData2")
+        spark
+          .range(0, 1000, 1, 10)
+          .select(
+            when('id < 250, 249)
+              .when('id >= 750, 1000)
+              .otherwise('id).as("key3"),
+            'id as "value3")
+          .createOrReplaceTempView("skewData3")
+        spark
+          .range(0, 1000, 1, 10)
+          .select(
+            when('id < 250, 249)
+              .otherwise('id).as("key4"),
+            'id as "value4")
+          .createOrReplaceTempView("skewData4")
+
+        def checkSkewJoin(
+            joins: Seq[SortMergeJoinExec],
+            leftSkewNum: Int,
+            rightSkewNum: Int): Unit = {
+          assert(joins.size == 1 && joins.head.isSkewJoin)
+          assert(joins.head.left.collect {
+            case r: CustomShuffleReaderExec => r
+          }.head.partitionSpecs.collect {
+            case p: PartialReducerPartitionSpec => p.reducerIndex
+          }.distinct.length == leftSkewNum)
+          assert(joins.head.right.collect {
+            case r: CustomShuffleReaderExec => r
+          }.head.partitionSpecs.collect {
+            case p: PartialReducerPartitionSpec => p.reducerIndex
+          }.distinct.length == rightSkewNum)
+        }
+
+        // skewed inner join optimization with union (not union all)
+        val (_, innerAdaptivePlan) = runAdaptiveAndVerifyResult(
+          "SELECT * FROM skewData1 join skewData2 ON key1 = key2 " +
+            "UNION SELECT * FROM skewData3 join skewData4 ON key3 = key4")
+        innerAdaptivePlan transformUp {
+          case u@UnionExec(c) =>
+            val innerSmj1 = findTopLevelSortMergeJoin(c.head)
+            val innerSmj2 = findTopLevelSortMergeJoin(c.tail.head)
+            checkSkewJoin(innerSmj1, 2, 1)
+            checkSkewJoin(innerSmj2, 2, 1)
+            u
+        }
+
+        // skewed left outer join optimization with union all
+        val (_, leftAdaptivePlan) = runAdaptiveAndVerifyResult(
+          "SELECT * FROM skewData1 left outer join skewData2 ON key1 = key2 " +
+            "UNION ALL SELECT * FROM skewData3 left outer join skewData4 ON key3 = key4")
+        leftAdaptivePlan transformUp {
+          case u@UnionExec(c) =>
+            val leftSmj1 = findTopLevelSortMergeJoin(c.head)
+            val leftSmj2 = findTopLevelSortMergeJoin(c.tail.head)
+            checkSkewJoin(leftSmj1, 2, 0)
+            checkSkewJoin(leftSmj2, 2, 0)
+            u
+        }
+
+        // skewed right outer join optimization with union all
+        val (_, rightAdaptivePlan) = runAdaptiveAndVerifyResult(
+          "SELECT * FROM skewData1 right outer join skewData2 ON key1 = key2 " +
+            "UNION ALL SELECT * FROM skewData3 right outer join skewData4 ON key3 = key4")
+        rightAdaptivePlan transformUp {
+          case u@UnionExec(c) =>
+            val rightSmj1 = findTopLevelSortMergeJoin(c.head)
+            val rightSmj2 = findTopLevelSortMergeJoin(c.tail.head)
+            checkSkewJoin(rightSmj1, 0, 1)
+            checkSkewJoin(rightSmj2, 0, 1)
+            u
+        }
+      }
+    }
+  }
+
   test("SPARK-30291: AQE should catch the exceptions when doing materialize") {
     withSQLConf(
       SQLConf.ADAPTIVE_EXECUTION_ENABLED.key -> "true") {