diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminManager.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminManager.java
index fc826e4fa4149..6b176b4acea7f 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminManager.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminManager.java
@@ -751,8 +751,10 @@ private void processBlocksInternal(
         // Remove the block from the list if it's no longer in the block map,
         // e.g. the containing file has been deleted
         if (blockManager.blocksMap.getStoredBlock(block) == null) {
-          LOG.trace("Removing unknown block {}", block);
-          it.remove();
+          if (pruneReliableBlocks) {
+            LOG.trace("Removing unknown block {}", block);
+            it.remove();
+          }
           continue;
         }
 
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestDecommission.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestDecommission.java
index a9e2bd593f2bf..39b93f281a468 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestDecommission.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestDecommission.java
@@ -53,6 +53,7 @@
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.hdfs.client.HdfsDataInputStream;
 import org.apache.hadoop.hdfs.client.HdfsDataOutputStream;
+import org.apache.hadoop.hdfs.protocol.Block;
 import org.apache.hadoop.hdfs.protocol.DatanodeID;
 import org.apache.hadoop.hdfs.protocol.DatanodeInfo;
 import org.apache.hadoop.hdfs.protocol.DatanodeInfo.AdminStates;
@@ -61,6 +62,7 @@
 import org.apache.hadoop.hdfs.protocol.LocatedBlock;
 import org.apache.hadoop.hdfs.protocol.LocatedBlocks;
 import org.apache.hadoop.hdfs.server.blockmanagement.BlockInfo;
+import org.apache.hadoop.hdfs.server.blockmanagement.BlockInfoContiguous;
 import org.apache.hadoop.hdfs.server.blockmanagement.BlockManager;
 import org.apache.hadoop.hdfs.server.blockmanagement.BlockManagerTestUtil;
 import org.apache.hadoop.hdfs.server.blockmanagement.DatanodeDescriptor;
@@ -668,6 +670,21 @@ public void testDecommissionWithOpenfile()
     fdos.close();
   }
 
+  @Test(timeout = 20000)
+  public void testDecommissionWithUnknownBlock() throws IOException {
+    startCluster(1, 3);
+
+    FSNamesystem ns = getCluster().getNamesystem(0);
+    DatanodeManager datanodeManager = ns.getBlockManager().getDatanodeManager();
+
+    BlockInfo blk = new BlockInfoContiguous(new Block(1L), (short) 1);
+    DatanodeDescriptor dn = datanodeManager.getDatanodes().iterator().next();
+    dn.getStorageInfos()[0].addBlock(blk, blk);
+
+    datanodeManager.getDatanodeAdminManager().startDecommission(dn);
+    waitNodeState(dn, DatanodeInfo.AdminStates.DECOMMISSIONED);
+  }
+
   private static String scanIntoString(final ByteArrayOutputStream baos) {
     final TextStringBuilder sb = new TextStringBuilder();
     final Scanner scanner = new Scanner(baos.toString());