diff --git a/sql/core/src/main/java/org/apache/spark/sql/execution/datasources/parquet/ParquetVectorUpdaterFactory.java b/sql/core/src/main/java/org/apache/spark/sql/execution/datasources/parquet/ParquetVectorUpdaterFactory.java
index 8c4fe20853879..523c4e1492fc6 100644
--- a/sql/core/src/main/java/org/apache/spark/sql/execution/datasources/parquet/ParquetVectorUpdaterFactory.java
+++ b/sql/core/src/main/java/org/apache/spark/sql/execution/datasources/parquet/ParquetVectorUpdaterFactory.java
@@ -495,7 +495,9 @@ public void decodeSingleDictionaryId(
         WritableColumnVector values,
         WritableColumnVector dictionaryIds,
         Dictionary dictionary) {
-      values.putLong(offset, dictionary.decodeToLong(dictionaryIds.getDictId(offset)));
+      // 32-bit Decimal target (precision <= 9) is stored in `intData`; `longData` is
+      // unallocated, so use `putInt` with the same narrowing cast as `readValue`/`readValues`.
+      values.putInt(offset, (int) dictionary.decodeToLong(dictionaryIds.getDictId(offset)));
     }
   }
 
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetIOSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetIOSuite.scala
index 8ed9ef1630eb2..710ab21090ff9 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetIOSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetIOSuite.scala
@@ -1593,6 +1593,46 @@ class ParquetIOSuite extends QueryTest with ParquetTest with SharedSparkSession
       }
     }
   }
+
+  test("SPARK-56872: INT64 DECIMAL into 32-bit Decimal column with dictionary fallback") {
+    // `DowncastLongUpdater.decodeSingleDictionaryId` only runs when the vectorized reader has
+    // to eagerly drain buffered dictionary IDs, which happens when parquet-mr writes the
+    // column as a mix of dictionary-encoded and PLAIN pages. The mix-cardinality values below
+    // (4-value pool + unique-per-row) force that fallback; uniformly low- or high-cardinality
+    // data bypasses the path. INT64 DECIMAL(p<=9) is built via parquet-mr's low-level writer
+    // because Spark's own writer emits INT32 for that case.
+    val schema = MessageTypeParser.parseMessageType(
+      """message root {
+        |  required int64 v (DECIMAL(9, 2));
+        |}""".stripMargin)
+    def unscaledAt(i: Int): Long = i % 5 match {
+      case 0 => -999999999L
+      case 1 => -1L
+      case 2 => 0L
+      case 3 => 999999999L
+      case _ => i.toLong * 13L - 7L
+    }
+    withTempDir { dir =>
+      val tablePath = new Path(s"${dir.getCanonicalPath}/dec.parquet")
+      val writer = createParquetWriter(schema, tablePath, dictionaryEnabled = true)
+      val numRecords = 5000
+      (0 until numRecords).foreach { i =>
+        val record = new SimpleGroup(schema)
+        record.add(0, unscaledAt(i))
+        writer.write(record)
+      }
+      writer.close()
+
+      withAllParquetReaders {
+        val readSchema = new StructType().add("v", DecimalType(9, 2), nullable = false)
+        val df = spark.read.schema(readSchema).parquet(tablePath.toString)
+        val expected = (0 until numRecords).map { i =>
+          Row(java.math.BigDecimal.valueOf(unscaledAt(i), 2))
+        }
+        checkAnswer(df, expected)
+      }
+    }
+  }
 }
 
 class JobCommitFailureParquetOutputCommitter(outputPath: Path, context: TaskAttemptContext)