diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -2640,6 +2640,31 @@
     with repo.wlock():
         repo.setparents(node1, node2)
 
+@command('debugsidedata', cmdutil.debugrevlogopts, _('-c|-m|FILE REV'))
+def debugsidedata(ui, repo, file_, rev=None, **opts):
+    """dump the side data for a cl/manifest/file revision"""
+    opts = pycompat.byteskwargs(opts)
+    if opts.get('changelog') or opts.get('manifest') or opts.get('dir'):
+        if rev is not None:
+            raise error.CommandError('debugdata', _('invalid arguments'))
+        file_, rev = None, file_
+    elif rev is None:
+        raise error.CommandError('debugdata', _('invalid arguments'))
+    r = cmdutil.openstorage(repo, 'debugdata', file_, opts)
+    r = getattr(r, '_revlog', r)
+    try:
+        sidedata = r.sidedata(r.lookup(rev))
+    except KeyError:
+        raise error.Abort(_('invalid revision identifier %s') % rev)
+    if sidedata:
+        sidedata = list(sidedata.items())
+        sidedata.sort()
+        ui.write(('%d sidedata entries\n' % len(sidedata)))
+        for key, value in sidedata:
+            ui.write((' entry-%04o size %d\n' % (key, len(value))))
+            if ui.verbose:
+                ui.write(('  %s\n' % repr(value)))
+
 @command('debugssl', [], '[SOURCE]', optionalrepo=True)
 def debugssl(ui, repo, source=None, **opts):
     '''test a secure connection to a server
diff --git a/tests/test-completion.t b/tests/test-completion.t
--- a/tests/test-completion.t
+++ b/tests/test-completion.t
@@ -124,6 +124,7 @@
   debugrevspec
   debugserve
   debugsetparents
+  debugsidedata
   debugssl
   debugsub
   debugsuccessorssets
@@ -305,6 +306,7 @@
   debugrevspec: optimize, show-revs, show-set, show-stage, no-optimized, verify-optimized
   debugserve: sshstdio, logiofd, logiofile
   debugsetparents: 
+  debugsidedata: changelog, manifest, dir
   debugssl: 
   debugsub: rev
   debugsuccessorssets: closest
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -1048,6 +1048,8 @@
    debugserve    run a server with advanced settings
    debugsetparents
                  manually set the parents of the current working directory
+   debugsidedata
+                 dump the side data for a cl/manifest/file revision
    debugssl      test a secure connection to a server
    debugsub      (no help text available)
    debugsuccessorssets
diff --git a/tests/test-sidedata.t b/tests/test-sidedata.t
--- a/tests/test-sidedata.t
+++ b/tests/test-sidedata.t
@@ -21,6 +21,25 @@
   $ echo xxx >> a
   $ hg commit -m aa
 
+  $ hg debugsidedata -c 0
+  2 sidedata entries
+   entry-0001 size 4
+   entry-0002 size 32
+  $ hg debugsidedata -c 1 -v
+  2 sidedata entries
+   entry-0001 size 4
+    '\x00\x00\x006'
+   entry-0002 size 32
+    '\x98\t\xf9\xc4v\xf0\xc5P\x90\xf7wRf\xe8\xe27e\xfc\xc1\x93\xa4\x96\xd0\x1d\x97\xaaG\x1d\xd7t\xfa\xde'
+  $ hg debugsidedata -m 2
+  2 sidedata entries
+   entry-0001 size 4
+   entry-0002 size 32
+  $ hg debugsidedata a  1
+  2 sidedata entries
+   entry-0001 size 4
+   entry-0002 size 32
+
 Check upgrade behavior
 ======================