diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -189,6 +189,13 @@
         self.activebookmark = None
         self.destmap = {}
         self.skipped = set()
+        self.parentmap = {}
+        for pm in opts.get(b'parentmap', []):
+            if pm.count(':') != 1:
+                raise error.Abort(_(b"--parentmap takes a pair of revisions"
+                                    b"separated by colon"))
+            oldp, newp = pm.split(":")
+            self.parentmap[int(oldp)] = int(newp)
 
         self.collapsef = opts.get(b'collapse', False)
         self.collapsemsg = cmdutil.logmessage(ui, opts)
@@ -614,6 +621,7 @@
                 self.state,
                 self.skipped,
                 self.obsoletenotrebased,
+                self.parentmap,
             )
             if not self.inmemory and len(repo[None].parents()) == 2:
                 repo.ui.debug(b'resuming interrupted rebase\n')
@@ -692,6 +700,7 @@
                 self.state,
                 self.skipped,
                 self.obsoletenotrebased,
+                self.parentmap,
             )
             editopt = opts.get(b'edit')
             editform = b'rebase.collapse'
@@ -870,6 +879,9 @@
             _(b'read collapse commit message from file'),
             _(b'FILE'),
         ),
+        (b'p', b'parentmap', [],
+            _(b'map old parent to new parent (ADVANCED)'),
+            _(b'REV:REV')),
         (b'k', b'keep', False, _(b'keep original changesets')),
         (b'', b'keepbranches', False, _(b'keep original branch names')),
         (b'D', b'detach', False, _(b'(DEPRECATED)')),
@@ -1649,7 +1661,7 @@
             yield nodemap[s]
 
 
-def defineparents(repo, rev, destmap, state, skipped, obsskipped):
+def defineparents(repo, rev, destmap, state, skipped, obsskipped, parentmap):
     """Return new parents and optionally a merge base for rev being rebased
 
     The destination specified by "dest" cannot always be used directly because
@@ -1707,6 +1719,8 @@
                 np = dests[i]
             elif p in state and state[p] > 0:
                 np = state[p]
+            elif p in parentmap:
+                np = parentmap[p]
 
             # "bases" only record "special" merge bases that cannot be
             # calculated from changelog DAG (i.e. isancestor(p, np) is False).
diff --git a/tests/test-rebase-scenario-global.t b/tests/test-rebase-scenario-global.t
--- a/tests/test-rebase-scenario-global.t
+++ b/tests/test-rebase-scenario-global.t
@@ -982,3 +982,81 @@
 
   $ [ -f .hg/rebasestate ] && echo 'WRONG: rebasestate should not exist'
   [1]
+
+Testing rebase of a merge unto a different parent
+
+  $ cd $TESTTNO
+  $ hg init reparent
+  $ cd reparent
+  $ hg debugbuilddag '+2*2 /2 *4'
+  $ hg log -G
+  o  changeset:   4:e7bd5218ca15
+  |  tag:         tip
+  |  parent:      0:1ea73414a91b
+  |  user:        debugbuilddag
+  |  date:        Thu Jan 01 00:00:04 1970 +0000
+  |  summary:     r4
+  |
+  | o    changeset:   3:2b6d669947cd
+  | |\   parent:      2:fa942426a6fd
+  | | |  parent:      1:66f7d451a68b
+  | | |  user:        debugbuilddag
+  | | |  date:        Thu Jan 01 00:00:03 1970 +0000
+  | | |  summary:     r3
+  | | |
+  +---o  changeset:   2:fa942426a6fd
+  | |    parent:      0:1ea73414a91b
+  | |    user:        debugbuilddag
+  | |    date:        Thu Jan 01 00:00:02 1970 +0000
+  | |    summary:     r2
+  | |
+  | o  changeset:   1:66f7d451a68b
+  |/   user:        debugbuilddag
+  |    date:        Thu Jan 01 00:00:01 1970 +0000
+  |    summary:     r1
+  |
+  o  changeset:   0:1ea73414a91b
+     user:        debugbuilddag
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     r0
+  
+  $ hg rebase -r 3 -d 4
+  rebasing 3:2b6d669947cd "r3"
+  abort: cannot rebase 3:2b6d669947cd without moving at least one of its parents
+  [255]
+  $ hg rebase --abort
+  rebase aborted
+  $ hg rebase --parentmap 1:4 -r 3 -d 4
+  rebasing 3:2b6d669947cd "r3"
+  saved backup bundle to $TESTTMP/reparent/.hg/strip-backup/2b6d669947cd-f1a6a874-rebase.hg
+  $ hg log -G
+  o    changeset:   4:53e482d07281
+  |\   tag:         tip
+  | |  parent:      3:e7bd5218ca15
+  | |  parent:      2:fa942426a6fd
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:03 1970 +0000
+  | |  summary:     r3
+  | |
+  | o  changeset:   3:e7bd5218ca15
+  | |  parent:      0:1ea73414a91b
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:04 1970 +0000
+  | |  summary:     r4
+  | |
+  o |  changeset:   2:fa942426a6fd
+  |/   parent:      0:1ea73414a91b
+  |    user:        debugbuilddag
+  |    date:        Thu Jan 01 00:00:02 1970 +0000
+  |    summary:     r2
+  |
+  | o  changeset:   1:66f7d451a68b
+  |/   user:        debugbuilddag
+  |    date:        Thu Jan 01 00:00:01 1970 +0000
+  |    summary:     r1
+  |
+  o  changeset:   0:1ea73414a91b
+     user:        debugbuilddag
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     r0
+