diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/diskbalancer/command/HelpCommand.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/diskbalancer/command/HelpCommand.java
index ec180538a9843..a3b6df61bd784 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/diskbalancer/command/HelpCommand.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/diskbalancer/command/HelpCommand.java
@@ -56,6 +56,15 @@ public void execute(CommandLine cmd) throws Exception {
     Preconditions.checkState(cmd.hasOption(DiskBalancerCLI.HELP));
     verifyCommandOptions(DiskBalancerCLI.HELP, cmd);
     String helpCommand = cmd.getOptionValue(DiskBalancerCLI.HELP);
+    if (helpCommand == null || helpCommand.isEmpty()) {
+      // With optionalArg(true), space-separated form "-help <cmd>" leaves the
+      // sub-command as a positional arg rather than the option's value.
+      // Fall back to the first leftover arg so "-help plan" works as expected.
+      String[] leftoverArgs = cmd.getArgs();
+      if (leftoverArgs != null && leftoverArgs.length > 0) {
+        helpCommand = leftoverArgs[0];
+      }
+    }
     if (helpCommand == null || helpCommand.isEmpty()) {
       this.printHelp();
       return;
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/diskbalancer/command/TestDiskBalancerCommand.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/diskbalancer/command/TestDiskBalancerCommand.java
index 7f9c05a6a721e..e88aac28ab72e 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/diskbalancer/command/TestDiskBalancerCommand.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/diskbalancer/command/TestDiskBalancerCommand.java
@@ -756,6 +756,33 @@ public void testHelpCommand() throws Exception {
     runCommand(cmdLine);
   }
 
+  /**
+   * HDFS-17809: "-help <command>" must display command-specific help text,
+   * not just the generic usage summary.
+   */
+  @Test
+  @Timeout(value = 60)
+  public void testHelpCommandWithSubCommand() throws Exception {
+    // Each sub-command help should contain its own option/keyword.
+    String[][] subCommands = {
+        {PLAN,    "plan"},
+        {EXECUTE, "execute"},
+        {QUERY,   "query"},
+        {CANCEL,  "cancel"},
+        {REPORT,  "report"},
+    };
+    for (String[] pair : subCommands) {
+      String subCmd = pair[0];
+      String expectedToken = pair[1];
+      String cmdLine = String.format("hdfs diskbalancer -%s %s", HELP, subCmd);
+      List<String> output = runCommand(cmdLine);
+      String joined = String.join("\n", output).toLowerCase();
+      assertTrue(joined.contains(expectedToken),
+          "Expected '" + expectedToken + "' in help output for '-help "
+              + subCmd + "', got:\n" + joined);
+    }
+  }
+
   @Test
   public void testPrintFullPathOfPlan()
       throws Exception {