diff --git a/core/src/main/java/org/apache/spark/shuffle/sort/ShuffleExternalSorter.java b/core/src/main/java/org/apache/spark/shuffle/sort/ShuffleExternalSorter.java
index f053135c4dbd7..0ca787d92412d 100644
--- a/core/src/main/java/org/apache/spark/shuffle/sort/ShuffleExternalSorter.java
+++ b/core/src/main/java/org/apache/spark/shuffle/sort/ShuffleExternalSorter.java
@@ -122,6 +122,13 @@ final class ShuffleExternalSorter extends MemoryConsumer implements ShuffleCheck
   // Checksum calculator for each partition. Empty when shuffle checksum disabled.
   private final Checksum[] partitionChecksums;
 
+  // Small writes to DiskBlockObjectWriter will be fairly inefficient. Since there doesn't seem to
+  // be an API to directly transfer bytes from managed memory to the disk writer, we buffer
+  // data through a byte array. This array does not need to be large enough to hold a single
+  // record;
+  // Lazily allocated on the first call to writeSortedFile() and reused across spills.
+  private byte[] writeBuffer;
+
   ShuffleExternalSorter(
       TaskMemoryManager memoryManager,
       BlockManager blockManager,
@@ -199,12 +206,6 @@ private void writeSortedFile(boolean isFinalFile) {
       writeMetricsToUse = new ShuffleWriteMetrics();
     }
 
-    // Small writes to DiskBlockObjectWriter will be fairly inefficient. Since there doesn't seem to
-    // be an API to directly transfer bytes from managed memory to the disk writer, we buffer
-    // data through a byte array. This array does not need to be large enough to hold a single
-    // record;
-    final byte[] writeBuffer = new byte[diskWriteBufferSize];
-
     // Because this output will be read during shuffle, its compression codec must be controlled by
     // spark.shuffle.compress instead of spark.shuffle.spill.compress, so we need to use
     // createTempShuffleBlock here; see SPARK-3426 for more details.
@@ -220,6 +221,10 @@ private void writeSortedFile(boolean isFinalFile) {
     // around this, we pass a dummy no-op serializer.
     final SerializerInstance ser = DummySerializerInstance.INSTANCE;
 
+    if (writeBuffer == null) {
+      writeBuffer = new byte[diskWriteBufferSize];
+    }
+
     int currentPartition = -1;
     final FileSegment committedSegment;
     try (DiskBlockObjectWriter writer =