diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala
index 2c57956de5bc..17f487947aec 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala
@@ -17,8 +17,6 @@
 
 package org.apache.spark.sql.execution.joins
 
-import scala.collection.mutable.ArrayBuffer
-
 import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.expressions._
@@ -266,7 +264,10 @@ case class SortMergeJoinExec(
             rightIter = RowIterator.fromScala(rightIter),
             boundCondition,
             leftNullRow,
-            rightNullRow)
+            rightNullRow,
+            inMemoryThreshold,
+            spillThreshold,
+            cleanupResources)
 
           new FullOuterIterator(
             smjScanner,
@@ -998,7 +999,10 @@ private class SortMergeFullOuterJoinScanner(
     rightIter: RowIterator,
     boundCondition: InternalRow => Boolean,
     leftNullRow: InternalRow,
-    rightNullRow: InternalRow)  {
+    rightNullRow: InternalRow,
+    inMemoryThreshold: Int,
+    spillThreshold: Int,
+    eagerCleanupResources: () => Unit)  {
   private[this] val joinedRow: JoinedRow = new JoinedRow()
   private[this] var leftRow: InternalRow = _
   private[this] var leftRowKey: InternalRow = _
@@ -1007,8 +1011,14 @@ private class SortMergeFullOuterJoinScanner(
 
   private[this] var leftIndex: Int = 0
   private[this] var rightIndex: Int = 0
-  private[this] val leftMatches: ArrayBuffer[InternalRow] = new ArrayBuffer[InternalRow]
-  private[this] val rightMatches: ArrayBuffer[InternalRow] = new ArrayBuffer[InternalRow]
+  private[this] var leftMatchesIterator: Iterator[UnsafeRow] = _
+  private[this] var rightMatchesIterator: Iterator[UnsafeRow] = _
+  private[this] var leftCurrentRow: InternalRow = _
+  private[this] var rightCurrentRow: InternalRow = _
+  private[this] val leftMatches: ExternalAppendOnlyUnsafeRowArray =
+    new ExternalAppendOnlyUnsafeRowArray(inMemoryThreshold, spillThreshold)
+  private[this] val rightMatches: ExternalAppendOnlyUnsafeRowArray =
+    new ExternalAppendOnlyUnsafeRowArray(inMemoryThreshold, spillThreshold)
   private[this] var leftMatched: BitSet = new BitSet(1)
   private[this] var rightMatched: BitSet = new BitSet(1)
 
@@ -1060,23 +1070,32 @@ private class SortMergeFullOuterJoinScanner(
     rightIndex = 0
 
     while (leftRowKey != null && keyOrdering.compare(leftRowKey, matchingKey) == 0) {
-      leftMatches += leftRow.copy()
+      leftMatches.add(leftRow.copy().asInstanceOf[UnsafeRow])
       advancedLeft()
     }
     while (rightRowKey != null && keyOrdering.compare(rightRowKey, matchingKey) == 0) {
-      rightMatches += rightRow.copy()
+      rightMatches.add(rightRow.copy().asInstanceOf[UnsafeRow])
       advancedRight()
     }
 
-    if (leftMatches.size <= leftMatched.capacity) {
-      leftMatched.clearUntil(leftMatches.size)
+    if (leftMatches.length > 0) {
+      leftMatchesIterator = leftMatches.generateIterator()
+      leftCurrentRow = leftMatchesIterator.next()
+    }
+    if (rightMatches.length > 0) {
+      rightMatchesIterator = rightMatches.generateIterator()
+      rightCurrentRow = rightMatchesIterator.next()
+    }
+
+    if (leftMatches.length <= leftMatched.capacity) {
+      leftMatched.clearUntil(leftMatches.length)
     } else {
-      leftMatched = new BitSet(leftMatches.size)
+      leftMatched = new BitSet(leftMatches.length)
     }
-    if (rightMatches.size <= rightMatched.capacity) {
-      rightMatched.clearUntil(rightMatches.size)
+    if (rightMatches.length <= rightMatched.capacity) {
+      rightMatched.clearUntil(rightMatches.length)
     } else {
-      rightMatched = new BitSet(rightMatches.size)
+      rightMatched = new BitSet(rightMatches.length)
     }
   }
 
@@ -1090,48 +1109,70 @@ private class SortMergeFullOuterJoinScanner(
    * @return true if a valid match is found, false otherwise.
    */
   private def scanNextInBuffered(): Boolean = {
-    while (leftIndex < leftMatches.size) {
-      while (rightIndex < rightMatches.size) {
-        joinedRow(leftMatches(leftIndex), rightMatches(rightIndex))
+    while (leftIndex < leftMatches.length) {
+      while (rightIndex < rightMatches.length) {
+        joinedRow(leftCurrentRow, rightCurrentRow)
         if (boundCondition(joinedRow)) {
           leftMatched.set(leftIndex)
           rightMatched.set(rightIndex)
           rightIndex += 1
+          nextRightRow()
           return true
         }
         rightIndex += 1
+        nextRightRow()
       }
       rightIndex = 0
+      if (rightMatches.length > 0) {
+        rightMatchesIterator = rightMatches.generateIterator()
+        rightCurrentRow = rightMatchesIterator.next()
+      }
       if (!leftMatched.get(leftIndex)) {
         // the left row has never matched any right row, join it with null row
-        joinedRow(leftMatches(leftIndex), rightNullRow)
+        joinedRow(leftCurrentRow, rightNullRow)
         leftIndex += 1
+        nextLeftRow()
         return true
       }
       leftIndex += 1
+      nextLeftRow()
     }
 
-    while (rightIndex < rightMatches.size) {
+    while (rightIndex < rightMatches.length) {
       if (!rightMatched.get(rightIndex)) {
         // the right row has never matched any left row, join it with null row
-        joinedRow(leftNullRow, rightMatches(rightIndex))
+        joinedRow(leftNullRow, rightCurrentRow)
         rightIndex += 1
+        nextRightRow()
         return true
       }
       rightIndex += 1
+      nextRightRow()
     }
 
     // There are no more valid matches in the left and right buffers
     false
   }
 
+  private def nextLeftRow(): Unit = {
+    if (leftIndex < leftMatches.length) {
+      leftCurrentRow = leftMatchesIterator.next()
+    }
+  }
+
+  private def nextRightRow(): Unit = {
+    if (rightIndex < rightMatches.length) {
+      rightCurrentRow = rightMatchesIterator.next()
+    }
+  }
+
   // --- Public methods --------------------------------------------------------------------------
 
   def getJoinedRow(): JoinedRow = joinedRow
 
   def advanceNext(): Boolean = {
     // If we already buffered some matching rows, use them directly
-    if (leftIndex <= leftMatches.size || rightIndex <= rightMatches.size) {
+    if (leftIndex <= leftMatches.length || rightIndex <= rightMatches.length) {
       if (scanNextInBuffered()) {
         return true
       }
@@ -1158,6 +1199,7 @@ private class SortMergeFullOuterJoinScanner(
       true
     } else {
       // Both iterators have been consumed
+      eagerCleanupResources()
       false
     }
   }
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/JoinSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/JoinSuite.scala
index fe6775cc7f9b..2cb8e4b0b400 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/JoinSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/JoinSuite.scala
@@ -791,9 +791,7 @@ class JoinSuite extends QueryTest with SharedSparkSession with AdaptiveSparkPlan
         )
       }
 
-      // FULL OUTER JOIN still does not use [[ExternalAppendOnlyUnsafeRowArray]]
-      // so should not cause any spill
-      assertNotSpilled(sparkContext, "full outer join") {
+      assertSpilled(sparkContext, "full outer join") {
         checkAnswer(
           sql(
             """