diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -1570,6 +1570,37 @@ coreconfigitem(
     default=False,
 )
 coreconfigitem(
+    b'partial-merge-tools',
+    b'.*',
+    default=None,
+    generic=True,
+    experimental=True,
+)
+coreconfigitem(
+    b'partial-merge-tools',
+    br'.*\.patterns',
+    default=dynamicdefault,
+    generic=True,
+    priority=-1,
+    experimental=True,
+)
+coreconfigitem(
+    b'partial-merge-tools',
+    br'.*\.executable$',
+    default=dynamicdefault,
+    generic=True,
+    priority=-1,
+    experimental=True,
+)
+coreconfigitem(
+    b'partial-merge-tools',
+    br'.*\.order',
+    default=0,
+    generic=True,
+    priority=-1,
+    experimental=True,
+)
+coreconfigitem(
     b'merge-tools',
     b'.*',
     default=None,
diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -1051,6 +1051,7 @@ def filemerge(repo, wctx, mynode, orig, 
             markerstyle = internalmarkerstyle
 
         if mergetype == fullmerge:
+            _run_partial_resolution_tools(repo, local, other, base)
             # conflict markers generated by premerge will use 'detailed'
             # settings if either ui.mergemarkers or the tool's mergemarkers
             # setting is 'detailed'. This way tools can have basic labels in
@@ -1115,6 +1116,62 @@ def filemerge(repo, wctx, mynode, orig, 
             backup.remove()
 
 
+def _run_partial_resolution_tools(repo, local, other, base):
+    """Runs partial-resolution tools on the three inputs and updates them."""
+    ui = repo.ui
+    # Tuples of (order, name, executable path)
+    tools = []
+    seen = set()
+    section = b"partial-merge-tools"
+    for k, v in ui.configitems(section):
+        name = k.split(b'.')[0]
+        if name in seen:
+            continue
+        patterns = ui.configlist(section, b'%s.patterns' % name, [])
+        is_match = True
+        if patterns:
+            m = match.match(repo.root, b'', patterns)
+            is_match = m(local.fctx.path())
+        if is_match:
+            order = ui.configint(section, b'%s.order' % name, 0)
+            executable = ui.config(section, b'%s.executable' % name, name)
+            tools.append((order, name, executable))
+
+    if not tools:
+        return
+    # Sort in configured order (first in tuple)
+    tools.sort()
+
+    files = [
+        (b"local", local.fctx.path(), local.text()),
+        (b"base", base.fctx.path(), base.text()),
+        (b"other", other.fctx.path(), other.text()),
+    ]
+
+    with _maketempfiles(files) as temppaths:
+        localpath, basepath, otherpath = temppaths
+
+        for order, name, executable in tools:
+            cmd = procutil.shellquote(executable)
+            # TODO: Allow the user to configure the command line using
+            # $local, $base, $other.
+            cmd = b'%s %s %s %s' % (cmd, localpath, basepath, otherpath)
+            r = ui.system(cmd, cwd=repo.root, blockedtag=b'partial-mergetool')
+            if r:
+                raise error.StateError(
+                    b'partial merge tool %s exited with code %d' % (name, r)
+                )
+            local_text = util.readfile(localpath)
+            other_text = util.readfile(otherpath)
+            if local_text == other_text:
+                # No need to run other tools if all conflicts have been resolved
+                break
+
+        local.set_text(local_text)
+        base.set_text(util.readfile(basepath))
+        other.set_text(other_text)
+
+
 def _haltmerge():
     msg = _(b'merge halted after failed merge (see hg resolve)')
     raise error.InterventionRequired(msg)
diff --git a/mercurial/simplemerge.py b/mercurial/simplemerge.py
--- a/mercurial/simplemerge.py
+++ b/mercurial/simplemerge.py
@@ -490,6 +490,9 @@ class MergeInput:
             self._text = self.fctx.decodeddata()
         return self._text
 
+    def set_text(self, text):
+        self._text = text
+
 
 def simplemerge(
     local,
diff --git a/tests/test-merge-partial-tool.t b/tests/test-merge-partial-tool.t
new file mode 100644
--- /dev/null
+++ b/tests/test-merge-partial-tool.t
@@ -0,0 +1,209 @@
+Test support for partial-resolution tools
+
+Create a tool that resolves conflicts after line 5 by simply dropping those
+lines (even if there are no conflicts there)
+  $ cat >> "$TESTTMP/head.sh" <<'EOF'
+  > #!/bin/sh
+  > for f in "$@"; do
+  >   head -5 $f > tmp
+  >   mv -f tmp $f
+  > done
+  > EOF
+  $ chmod +x "$TESTTMP/head.sh"
+...and another tool that keeps only the last 5 lines instead of the first 5.
+  $ cat >> "$TESTTMP/tail.sh" <<'EOF'
+  > #!/bin/sh
+  > for f in "$@"; do
+  >   tail -5 $f > tmp
+  >   mv -f tmp $f
+  > done
+  > EOF
+  $ chmod +x "$TESTTMP/tail.sh"
+
+Set up both tools to run on all patterns (the default), and let the `tail` tool
+run after the `head` tool, which means it will have no effect (we'll override it
+to test order later)
+  $ cat >> "$HGRCPATH" <<EOF
+  > [partial-merge-tools]
+  > head.executable=$TESTTMP/head.sh
+  > tail.executable=$TESTTMP/tail.sh
+  > tail.order=1
+  > EOF
+
+  $ make_commit() {
+  >   echo "$@" | xargs -n1 > file
+  >   hg add file 2> /dev/null
+  >   hg ci -m "$*"
+  > }
+
+
+Let a partial-resolution tool resolve some conflicts and leave other conflicts
+for the regular merge tool (:merge3 here)
+
+  $ hg init repo
+  $ cd repo
+  $ make_commit a b c d e f
+  $ make_commit a b2 c d e f2
+  $ hg up 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ make_commit a b3 c d e f3
+  created new head
+  $ hg merge 1 -t :merge3
+  merging file
+  warning: conflicts while merging file! (edit, then use 'hg resolve --mark')
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon
+  [1]
+  $ cat file
+  a
+  <<<<<<< working copy:    e11a49d4b620 - test: a b3 c d e f3
+  b3
+  ||||||| common ancestor: 8ae8bb9cc43a - test: a b c d e f
+  b
+  =======
+  b2
+  >>>>>>> merge rev:       fbc096a40cc5 - test: a b2 c d e f2
+  c
+  d
+  e
+
+
+With premerge=keep, the partial-resolution tools runs before and doesn't see
+the conflict markers
+
+  $ hg up -C 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cat >> .hg/hgrc <<EOF
+  > [merge-tools]
+  > my-local.executable = cat
+  > my-local.args = $local
+  > my-local.premerge = keep-merge3
+  > EOF
+  $ hg merge 1 -t my-local
+  merging file
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat file
+  a
+  <<<<<<< working copy:    e11a49d4b620 - test: a b3 c d e f3
+  b3
+  ||||||| common ancestor: 8ae8bb9cc43a - test: a b c d e f
+  b
+  =======
+  b2
+  >>>>>>> merge rev:       fbc096a40cc5 - test: a b2 c d e f2
+  c
+  d
+  e
+
+
+When a partial-resolution tool resolves all conflicts, the resolution should
+be recorded and the regular merge tool should not be invoked for the file.
+
+  $ hg up -C 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ make_commit a b c d e f2
+  created new head
+  $ hg up 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ make_commit a b c d e f3
+  created new head
+  $ hg merge 3 -t false
+  merging file
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat file
+  a
+  b
+  c
+  d
+  e
+
+
+Only tools whose patterns match are run. We make `head` not match here, so
+only `tail` should run
+
+  $ hg up -C 4
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge 3 -t :merge3 --config partial-merge-tools.head.patterns=other
+  merging file
+  warning: conflicts while merging file! (edit, then use 'hg resolve --mark')
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon
+  [1]
+  $ cat file
+  b
+  c
+  d
+  e
+  <<<<<<< working copy:    d57edaa6e21a - test: a b c d e f3
+  f3
+  ||||||| common ancestor: 8ae8bb9cc43a - test: a b c d e f
+  f
+  =======
+  f2
+  >>>>>>> merge rev:       8c217da987be - test: a b c d e f2
+
+
+If there are several matching tools, they are run in requested order. We move
+`head` after `tail` in order here so it has no effect (the conflict in "f" thus
+remains).
+
+  $ hg up -C 4
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge 3 -t :merge3 --config partial-merge-tools.head.order=2
+  merging file
+  warning: conflicts while merging file! (edit, then use 'hg resolve --mark')
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon
+  [1]
+  $ cat file
+  b
+  c
+  d
+  e
+  <<<<<<< working copy:    d57edaa6e21a - test: a b c d e f3
+  f3
+  ||||||| common ancestor: 8ae8bb9cc43a - test: a b c d e f
+  f
+  =======
+  f2
+  >>>>>>> merge rev:       8c217da987be - test: a b c d e f2
+
+
+When using "nomerge" tools (e.g. `:other`), the partial-resolution tools
+should not be run.
+
+  $ hg up -C 4
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge 3 -t :other
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat file
+  a
+  b
+  c
+  d
+  e
+  f2
+
+
+If a partial-resolution tool resolved some conflict and simplemerge can
+merge the rest, then the regular merge tool should not be used. Here we merge
+"a b c d e3 f3" with "a b2 c d e f2". The `head` tool resolves the conflict in
+"f" and the internal simplemerge merges the remaining changes in "b" and "e".
+
+  $ hg up -C 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ make_commit a b c d e3 f3
+  created new head
+  $ hg merge 1 -t false
+  merging file
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat file
+  a
+  b2
+  c
+  d
+  e3