diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index a3dc9d595f1f..8266f400b8cb 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -77,6 +77,9 @@ API Changes
 
 * GITHUB#12855: Remove deprecated DrillSideways#createDrillDownFacetsCollector extension method. (Greg Miller)
 
+* GITHUB#12980: Make FSTPostingsFormat to build FST off-heap. This PostingsFormat will now
+  create 2 FST files (tfp.meta and tfp.data) instead of a single one. (Anh Dung Bui)
+
 New Features
 ---------------------
 
diff --git a/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTPostingsFormat.java b/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTPostingsFormat.java
index 0a18b75ac003..6fbbafd9ad8b 100644
--- a/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTPostingsFormat.java
+++ b/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTPostingsFormat.java
@@ -28,10 +28,10 @@
 import org.apache.lucene.index.SegmentWriteState;
 import org.apache.lucene.util.IOUtils;
 
-/** FST term dict + Lucene50PBF */
+/** FST term dict + Lucene99PBF */
 public final class FSTPostingsFormat extends PostingsFormat {
   public FSTPostingsFormat() {
-    super("FST50");
+    super("FST99");
   }
 
   @Override
diff --git a/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsReader.java b/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsReader.java
index a5b26b77b6a6..22891d4b9f8d 100644
--- a/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsReader.java
+++ b/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsReader.java
@@ -65,7 +65,9 @@
 public class FSTTermsReader extends FieldsProducer {
   private final TreeMap<String, TermsReader> fields = new TreeMap<>();
   private final PostingsReaderBase postingsReader;
+  // IndexInput for FST metadata
   private final IndexInput fstMetaInput;
+  // IndexInput for FST data
   private final IndexInput fstDataInput;
 
   public FSTTermsReader(SegmentReadState state, PostingsReaderBase postingsReader)
diff --git a/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsWriter.java b/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsWriter.java
index e86ec33c323f..3addb4875a53 100644
--- a/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsWriter.java
+++ b/lucene/codecs/src/java/org/apache/lucene/codecs/memory/FSTTermsWriter.java
@@ -115,8 +115,10 @@ public class FSTTermsWriter extends FieldsConsumer {
 
   final PostingsWriterBase postingsWriter;
   final FieldInfos fieldInfos;
-  IndexOutput dataOut;
+  // IndexOutput for FST metadata
   IndexOutput metaOut;
+  // IndexOutput for FST data
+  IndexOutput dataOut;
   final int maxDoc;
   final List<FieldMetaData> fields = new ArrayList<>();