diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 3840454685e..7f731ccb130 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -23,6 +23,9 @@ API Changes
 * LUCENE-8014: Similarity.computeSlopFactor() and
   Similarity.computePayloadFactor() have been removed (Alan Woodward)
 
+* LUCENE-8049: IndexWriter.getMergingSegments()'s return type was changed from
+  Collection to Set to more accurately reflect it's nature. (David Smiley)
+
 Changes in Runtime Behavior
 
 * LUCENE-7837: Indices that were created before the previous major version
diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
index 44fb36ec7ad..7f47e42d45d 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
@@ -2285,9 +2285,9 @@ public class IndexWriter implements Closeable, TwoPhaseCommit, Accountable {
    *  (which you do when IndexWriter invokes the
    *  MergePolicy).
    *
-   *  <p>Do not alter the returned collection! */
-  public synchronized Collection<SegmentCommitInfo> getMergingSegments() {
-    return mergingSegments;
+   *  <p>The Set is unmodifiable. */
+  public synchronized Set<SegmentCommitInfo> getMergingSegments() {
+    return Collections.unmodifiableSet(mergingSegments);
   }
 
   /**
diff --git a/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java b/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java
index 4d0ce51d261..78025634b4a 100644
--- a/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java
+++ b/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java
@@ -19,10 +19,10 @@ package org.apache.lucene.index;
 
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Collection;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Set;
 
 /**
  * <p>This class implements a {@link MergePolicy} that tries
@@ -462,7 +462,7 @@ public abstract class LogMergePolicy extends MergePolicy {
     final List<SegmentInfoAndLevel> levels = new ArrayList<>(numSegments);
     final float norm = (float) Math.log(mergeFactor);
 
-    final Collection<SegmentCommitInfo> mergingSegments = writer.getMergingSegments();
+    final Set<SegmentCommitInfo> mergingSegments = writer.getMergingSegments();
 
     for(int i=0;i<numSegments;i++) {
       final SegmentCommitInfo info = infos.info(i);
diff --git a/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java b/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java
index b5185fc9aef..cdfdea6e5af 100644
--- a/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java
+++ b/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java
@@ -26,6 +26,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Set;
 
 /**
  *  Merges segments of approximately equal size, subject to
@@ -287,8 +288,8 @@ public class TieredMergePolicy extends MergePolicy {
     if (infos.size() == 0) {
       return null;
     }
-    final Collection<SegmentCommitInfo> merging = writer.getMergingSegments();
-    final Collection<SegmentCommitInfo> toBeMerged = new HashSet<>();
+    final Set<SegmentCommitInfo> merging = writer.getMergingSegments();
+    final Set<SegmentCommitInfo> toBeMerged = new HashSet<>();
 
     final List<SegmentCommitInfo> infosSorted = new ArrayList<>(infos.asList());
 
@@ -519,7 +520,7 @@ public class TieredMergePolicy extends MergePolicy {
 
     List<SegmentCommitInfo> eligible = new ArrayList<>();
     boolean forceMergeRunning = false;
-    final Collection<SegmentCommitInfo> merging = writer.getMergingSegments();
+    final Set<SegmentCommitInfo> merging = writer.getMergingSegments();
     boolean segmentIsOriginal = false;
     for(SegmentCommitInfo info : infos) {
       final Boolean isOriginal = segmentsToMerge.get(info);
@@ -594,7 +595,7 @@ public class TieredMergePolicy extends MergePolicy {
       message("findForcedDeletesMerges infos=" + writer.segString(infos) + " forceMergeDeletesPctAllowed=" + forceMergeDeletesPctAllowed, writer);
     }
     final List<SegmentCommitInfo> eligible = new ArrayList<>();
-    final Collection<SegmentCommitInfo> merging = writer.getMergingSegments();
+    final Set<SegmentCommitInfo> merging = writer.getMergingSegments();
     for(SegmentCommitInfo info : infos) {
       double pctDeletes = 100.*((double) writer.numDeletedDocs(info))/info.info.maxDoc();
       if (pctDeletes > forceMergeDeletesPctAllowed && !merging.contains(info)) {
diff --git a/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java b/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java
index f9fa601cc32..61efaa2adb4 100644
--- a/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java
+++ b/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java
@@ -18,11 +18,11 @@ package org.apache.lucene.index;
 
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Random;
+import java.util.Set;
 
 import org.apache.lucene.util.LuceneTestCase;
 import org.apache.lucene.util.TestUtil;
@@ -56,7 +56,7 @@ public class MockRandomMergePolicy extends MergePolicy {
     int numSegments = segmentInfos.size();
 
     List<SegmentCommitInfo> segments = new ArrayList<>();
-    final Collection<SegmentCommitInfo> merging = writer.getMergingSegments();
+    final Set<SegmentCommitInfo> merging = writer.getMergingSegments();
 
     for(SegmentCommitInfo sipc : segmentInfos) {
       if (!merging.contains(sipc)) {