diff --git a/hgext/fastexport.py b/hgext/fastexport.py
new file mode 100644
--- /dev/null
+++ b/hgext/fastexport.py
@@ -0,0 +1,221 @@
+# Copyright 2020 Joerg Sonnenberger <joerg@bec.de>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""export repositories as git fast-import stream"""
+
+# The format specification for fast-import streams can be found at
+# https://git-scm.com/docs/git-fast-import#_input_format
+
+from __future__ import absolute_import
+import re
+
+from mercurial.i18n import _
+from mercurial.node import hex, nullrev
+from mercurial.utils import stringutil
+from mercurial import (
+    cmdutil,
+    commands,
+    destutil,
+    error,
+    formatter,
+    graphmod,
+    logcmdutil,
+    phases,
+    pycompat,
+    registrar,
+    revset,
+    revsetlang,
+    scmutil,
+)
+from .convert import convcmd
+
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = b"ships-with-hg-core"
+
+cmdtable = {}
+command = registrar.command(cmdtable)
+
+GIT_PERSON_PROHIBITED = re.compile(b'[<>\n"]')
+GIT_EMAIL_PROHIBITED = re.compile(b"[<> \n]")
+
+
+def convert_to_git_user(authormap, user, rev):
+    mapped_user = authormap.get(user, user)
+    user_person = stringutil.person(mapped_user)
+    user_email = stringutil.email(mapped_user)
+    if GIT_EMAIL_PROHIBITED.match(user_email) or GIT_PERSON_PROHIBITED.match(
+        user_person
+    ):
+        raise error.Abort(
+            _(b"Unable to parse user into person and email for revision " + rev)
+        )
+    if user_person:
+        return b'"' + user_person + b'" <' + user_email + b'>'
+    else:
+        return b"<" + user_email + b">"
+
+
+def convert_to_git_date(date):
+    timestamp, utcoff = date
+    tzsign = b"+" if utcoff < 0 else b"-"
+    if utcoff % 60 != 0:
+        raise error.Abort(
+            _(b"UTC offset in %b is not an integer number of seconds") % (date,)
+        )
+    utcoff = abs(utcoff) // 60
+    tzh = utcoff // 60
+    tzmin = utcoff % 60
+    return b"%d " % int(timestamp) + tzsign + b"%02d%02d" % (tzh, tzmin)
+
+
+def convert_to_git_ref(branch):
+    # XXX filter/map depending on git restrictions
+    return b"refs/heads/" + branch
+
+
+def write_data(buf, data, skip_newline):
+    buf.append(b"data %d\n" % len(data))
+    buf.append(data)
+    if not skip_newline or data[-1:] != b"\n":
+        buf.append(b"\n")
+
+
+def export_commit(ui, repo, rev, marks, authormap):
+    ctx = repo[rev]
+    revid = ctx.hex()
+    if revid in marks:
+        ui.warn(_(b"warning: revision %s already exported, skipped\n") % revid)
+        return
+    parents = [p for p in ctx.parents() if p.rev() != nullrev]
+    for p in parents:
+        if p.hex() not in marks:
+            ui.warn(
+                _(b"warning: parent %s of %s has not been exported, skipped\n")
+                % (p, revid)
+            )
+            return
+
+    # For all files modified by the commit, check if they have already
+    # been exported and otherwise dump the blob with the new mark.
+    for fname in ctx.files():
+        if fname not in ctx:
+            continue
+        filectx = ctx.filectx(fname)
+        filerev = hex(filectx.filenode())
+        if filerev not in marks:
+            mark = len(marks) + 1
+            marks[filerev] = mark
+            data = filectx.data()
+            buf = [b"blob\n", b"mark :%d\n" % mark]
+            write_data(buf, data, False)
+            ui.write(*buf, keepprogressbar=True)
+            del buf
+
+    # Assign a mark for the current revision for references by
+    # latter merge commits.
+    mark = len(marks) + 1
+    marks[revid] = mark
+
+    ref = convert_to_git_ref(ctx.branch())
+    buf = [
+        b"commit %s\n" % ref,
+        b"mark :%d\n" % mark,
+        b"committer %s %s\n"
+        % (
+            convert_to_git_user(authormap, ctx.user(), revid),
+            convert_to_git_date(ctx.date()),
+        ),
+    ]
+    write_data(buf, ctx.description(), True)
+    if parents:
+        buf.append(b"from :%d\n" % marks[parents[0].hex()])
+    if len(parents) == 2:
+        buf.append(b"merge :%d\n" % marks[parents[1].hex()])
+        p0ctx = repo[parents[0]]
+        files = ctx.manifest().diff(p0ctx.manifest())
+    else:
+        files = ctx.files()
+    filebuf = []
+    for fname in files:
+        if fname not in ctx:
+            filebuf.append((fname, b"D %s\n" % fname))
+        else:
+            filectx = ctx.filectx(fname)
+            filerev = filectx.filenode()
+            fileperm = b"755" if filectx.isexec() else b"644"
+            changed = b"M %s :%d %s\n" % (fileperm, marks[hex(filerev)], fname)
+            filebuf.append((fname, changed))
+    filebuf.sort()
+    buf.extend(changed for (fname, changed) in filebuf)
+    del filebuf
+    buf.append(b"\n")
+    ui.write(*buf, keepprogressbar=True)
+    del buf
+
+
+isrev = re.compile(b"^[0-9a-f]{40}$")
+
+
+@command(
+    b"fastexport",
+    [
+        (b"r", b"rev", [], _(b"revisions to export"), _(b"REV")),
+        (b"i", b"import-marks", b"", _(b"old marks file to read"), _(b"FILE")),
+        (b"e", b"export-marks", b"", _(b"new marks file to write"), _(b"FILE")),
+        (b"A", b"authormap", b"", _(b"remap usernames using this file"), _(b"FILE")),
+    ],
+    _(b"[OPTION]... [REV]..."),
+    helpcategory=command.CATEGORY_IMPORT_EXPORT,
+)
+def fastexport(ui, repo, *revs, **opts):
+    """export repository as git fast-import stream
+
+    This command lets you dump a repository as a human-readable text stream.
+    It can be piped into corresponding import routines like "git fast-import".
+    Incremental dumps can be created by using marks files.
+    """
+    opts = pycompat.byteskwargs(opts)
+
+    revs += tuple(opts.get(b"rev", []))
+    if not revs:
+        revs = scmutil.revrange(repo, [b":"])
+    else:
+        revs = scmutil.revrange(repo, revs)
+    if not revs:
+        raise error.Abort(_(b"no revisions matched"))
+    authorfile = opts.get(b"authormap")
+    if authorfile:
+        authormap = convcmd.readauthormap(ui, authorfile)
+    else:
+        authormap = {}
+
+    import_marks = opts.get(b"import_marks")
+    marks = {}
+    if import_marks:
+        with open(import_marks, "rb") as import_marks_file:
+            for line in import_marks_file:
+                line = line.strip()
+                if not isrev.match(line) or line in marks:
+                    raise error.Abort(_(b"Corrupted marks file"))
+                marks[line] = len(marks) + 1
+
+    revs.sort()
+    with ui.makeprogress(
+        _(b"exporting"), unit=_(b"revisions"), total=len(revs)
+    ) as progress:
+        for rev in revs:
+            export_commit(ui, repo, rev, marks, authormap)
+            progress.increment()
+
+    export_marks = opts.get(b"export_marks")
+    if export_marks:
+        with open(export_marks, "wb") as export_marks_file:
+            output_marks = [None] * len(marks)
+            for k, v in marks.items():
+                output_marks[v - 1] = k
+            for k in output_marks:
+                export_marks_file.write(k + b"\n")
diff --git a/tests/test-fastexport.t b/tests/test-fastexport.t
new file mode 100644
--- /dev/null
+++ b/tests/test-fastexport.t
@@ -0,0 +1,855 @@
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > fastexport=
+  > EOF
+
+  $ hg init
+
+  $ hg debugbuilddag -mon '+2:tbase @name1 +3:thead1 <tbase @name2 +4:thead2 @both /thead1 +2:tmaintip'
+
+  $ hg up -r 10
+  13 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg rm nf10
+  $ hg commit -u debugbuilddag --date 'Thu Jan 01 00:00:12 1970 +0000' -m r12
+  created new head
+  $ hg up -r 11
+  4 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge -r 12
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg commit -m debugbuilddag --date 'Thu Jan 01 00:00:13 1970 +0000'
+
+  $ hg log -G
+  @    changeset:   13:e5c379648af4
+  |\   branch:      both
+  | |  tag:         tip
+  | |  parent:      11:2cbd52c10e88
+  | |  parent:      12:4f31c9604af6
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:13 1970 +0000
+  | |  summary:     debugbuilddag
+  | |
+  | o  changeset:   12:4f31c9604af6
+  | |  branch:      both
+  | |  parent:      10:9220596cb068
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:12 1970 +0000
+  | |  summary:     r12
+  | |
+  o |  changeset:   11:2cbd52c10e88
+  |/   branch:      both
+  |    tag:         tmaintip
+  |    user:        debugbuilddag
+  |    date:        Thu Jan 01 00:00:11 1970 +0000
+  |    summary:     r11
+  |
+  o  changeset:   10:9220596cb068
+  |  branch:      both
+  |  user:        debugbuilddag
+  |  date:        Thu Jan 01 00:00:10 1970 +0000
+  |  summary:     r10
+  |
+  o    changeset:   9:0767d147d86e
+  |\   branch:      both
+  | |  parent:      8:0d0219415f18
+  | |  parent:      4:e8bc3a6ab9ae
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:09 1970 +0000
+  | |  summary:     r9
+  | |
+  | o  changeset:   8:0d0219415f18
+  | |  branch:      name2
+  | |  tag:         thead2
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:08 1970 +0000
+  | |  summary:     r8
+  | |
+  | o  changeset:   7:82c6c8b3ac68
+  | |  branch:      name2
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:07 1970 +0000
+  | |  summary:     r7
+  | |
+  | o  changeset:   6:94093a13175f
+  | |  branch:      name2
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:06 1970 +0000
+  | |  summary:     r6
+  | |
+  | o  changeset:   5:4baee2f72e9e
+  | |  branch:      name2
+  | |  parent:      1:bf4022f1addd
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:05 1970 +0000
+  | |  summary:     r5
+  | |
+  o |  changeset:   4:e8bc3a6ab9ae
+  | |  branch:      name1
+  | |  tag:         thead1
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:04 1970 +0000
+  | |  summary:     r4
+  | |
+  o |  changeset:   3:46148e496a8a
+  | |  branch:      name1
+  | |  user:        debugbuilddag
+  | |  date:        Thu Jan 01 00:00:03 1970 +0000
+  | |  summary:     r3
+  | |
+  o |  changeset:   2:29863c4219cd
+  |/   branch:      name1
+  |    user:        debugbuilddag
+  |    date:        Thu Jan 01 00:00:02 1970 +0000
+  |    summary:     r2
+  |
+  o  changeset:   1:bf4022f1addd
+  |  tag:         tbase
+  |  user:        debugbuilddag
+  |  date:        Thu Jan 01 00:00:01 1970 +0000
+  |  summary:     r1
+  |
+  o  changeset:   0:ae6ae30a671b
+     user:        debugbuilddag
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     r0
+  
+
+  $ hg fastexport --export-marks fastexport.marks
+  blob
+  mark :1
+  data 65
+  0 r0
+  1
+  2
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+  4
+  5
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+  
+  blob
+  mark :2
+  data 3
+  r0
+  
+  commit refs/heads/default
+  mark :3
+  committer "debugbuilddag" <debugbuilddag> 0 -0000
+  data 2
+  r0
+  M 644 :1 mf
+  M 644 :2 nf0
+  M 644 :2 of
+  
+  blob
+  mark :4
+  data 68
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+  
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+  mark :5
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+  r1
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+  mark :39
+  committer "debugbuilddag" <debugbuilddag> 9 -0000
+  data 2
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+  from :35
+  merge :19
+  M 644 :36 mf
+  M 644 :9 nf2
+  M 644 :13 nf3
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+  mark :40
+  data 96
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+  M 644 :46 of
+  
+  commit refs/heads/both
+  mark :48
+  committer "debugbuilddag" <debugbuilddag> 12 -0000
+  data 3
+  r12
+  from :43
+  D nf10
+  
+  commit refs/heads/both
+  mark :49
+  committer "test" <test> 13 -0000
+  data 13
+  debugbuilddag
+  from :47
+  merge :48
+  D nf10
+  
+  $ cat fastexport.marks
+  e1767c7564f83127d75331428473dd0512b36cc6
+  2c436e3f677d989438ddd9a7e5e4d56e016dfd35
+  ae6ae30a671be09096aaaf51217b3691eec0eee0
+  016f8fd6128ac4bd19ec5a6ae128dadc3873b13f
+  a0e6fc91007068df3bc60f46ce0a893a73189b54
+  1a085e1daf625e186ee0064c64ff41731a901f24
+  bf4022f1addd28523fb1122ac6166a29da58d34c
+  2c45ad1c720111830380baa89a6a16cae1bef688
+  180506669a19f4b8317009fc6fa0043966d1ffb4
+  1ebc486e6a5c2c8ca7e531cf0b63dfcc071ec324
+  29863c4219cd68e0f57aecd5ffc12ba83313f26b
+  d20e5eeac6991189eefad45cd8ea0f6a32ce8122
+  710c4580a600b8aadc63fa3d7bb0fab71b127c04
+  fa27314b56d7b6f90c1caeebb2a74730b3747574
+  46148e496a8a75fde9e203b1ded69ec99289af27
+  e5548c667d7eeb6c326e723c579888341329c9fe
+  3c1407305701051cbed9f9cb9a68bdfb5997c235
+  e2ed51893b0a54bd7fef5a406a0c489d668f19c3
+  e8bc3a6ab9aef589f5db504f401953449a3c3a10
+  558f3a23efc0a1a972e14d5314a65918791b77be
+  0dbd89c185f53a1727c54cd1ce256482fa23968e
+  f84faeb138605b36d74324c6d0ea76a9099c3567
+  4baee2f72e9eeae2aef5b9e1ec416020090672ef
+  412c5793886eaaabb31debd36695f6215a719865
+  a0eafc60760d32b690564b8588ba042cc63e0c74
+  a53842517de32d2f926c38a170c29dc90ae3348a
+  94093a13175f1cfcbbfddaa0ceafbd3a89784b91
+  d2f0d76af0be0da17ec88190215eadb8706689ab
+  639939af794373d6c2ab12c2ef637cd220174389
+  cc8921e2b19a88147643ea825459ffa140e3d704
+  82c6c8b3ac6873fadd9083323b02cc6a53659130
+  c6cc0b14a3e6e61906242d6fce28b9510c9f9208
+  093593169cb4716f94e52ed7561bb84b36b7eb9d
+  034df75dc138e7507e061d26170b4c44321a5d92
+  0d0219415f18c43636163fff4160f41600951a25
+  f13693f6e6052eeb189521945fef56892e812fdb
+  1239c633b8a7a7283825dba9171bf285e5790852
+  34b655bd51e8573b8e85c1c1476a94d8573babef
+  0767d147d86e1546593bda50f1e11276c0ac8f1a
+  284ca43bbbe82e89c0f1d977e8ac6cfb969c05ec
+  15315ab9e272ec81ae8d847996e5bdecd5635b0b
+  78c10aaf21f49d518c7ccb8318c29abb5d4e5db7
+  9220596cb068dfc73e2f7e695dc8ad0858a936db
+  32abd0da49b7c7ee756298fc46a15584d6aedc99
+  33fbc651630ffa7ccbebfe4eb91320a873e7291c
+  868d828870663d075cdcff502d26cf8445ce068e
+  2cbd52c10e88ce604402dc83a869ec4f07765b3d
+  4f31c9604af676986343d775b05695f535e8db5e
+  e5c379648af4c9fa3b5546ab7ee6e61a36082830
+
+  $ hg fastexport --export-marks fastexport.marks2 -r 0
+  blob
+  mark :1
+  data 65
+  0 r0
+  1
+  2
+  3
+  4
+  5
+  6
+  7
+  8
+  9
+  10
+  11
+  12
+  13
+  14
+  15
+  16
+  17
+  18
+  19
+  20
+  21
+  22
+  23
+  
+  blob
+  mark :2
+  data 3
+  r0
+  
+  commit refs/heads/default
+  mark :3
+  committer "debugbuilddag" <debugbuilddag> 0 -0000
+  data 2
+  r0
+  M 644 :1 mf
+  M 644 :2 nf0
+  M 644 :2 of
+  
+  $ cat fastexport.marks2
+  e1767c7564f83127d75331428473dd0512b36cc6
+  2c436e3f677d989438ddd9a7e5e4d56e016dfd35
+  ae6ae30a671be09096aaaf51217b3691eec0eee0
+  $ hg fastexport --import-marks fastexport.marks2 -r 1
+  blob
+  mark :4
+  data 68
+  0 r0
+  1
+  2 r1
+  3
+  4
+  5
+  6
+  7
+  8
+  9
+  10
+  11
+  12
+  13
+  14
+  15
+  16
+  17
+  18
+  19
+  20
+  21
+  22
+  23
+  
+  blob
+  mark :5
+  data 3
+  r1
+  
+  blob
+  mark :6
+  data 3
+  r1
+  
+  commit refs/heads/default
+  mark :7
+  committer "debugbuilddag" <debugbuilddag> 1 -0000
+  data 2
+  r1
+  from :3
+  M 644 :4 mf
+  M 644 :5 nf1
+  M 644 :6 of
+  
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -364,6 +364,7 @@
        eol           automatically manage newlines in repository files
        extdiff       command to allow external programs to compare revisions
        factotum      http authentication with factotum
+       fastexport    export repositories as git fast-import stream
        githelp       try mapping git commands to Mercurial commands
        gpg           commands to sign and verify changesets
        hgk           browse the repository in a graphical way