diff --git a/extensions/MozChangeField/Extension.pm b/extensions/MozChangeField/Extension.pm
index 6e2dbffdae..4bb68085d4 100644
--- a/extensions/MozChangeField/Extension.pm
+++ b/extensions/MozChangeField/Extension.pm
@@ -17,6 +17,7 @@ use Bugzilla::Constants;
 use Bugzilla::Logging;
 
 use Bugzilla::Extension::MozChangeField::Pre::CanConfirm;
+use Bugzilla::Extension::MozChangeField::Pre::CheckinNeededTB;
 use Bugzilla::Extension::MozChangeField::Pre::CommentClosedBugs;
 use Bugzilla::Extension::MozChangeField::Pre::CustomField;
 use Bugzilla::Extension::MozChangeField::Pre::Graveyard;
@@ -25,6 +26,7 @@ use Bugzilla::Extension::MozChangeField::Pre::TypePriSevEditbugs;
 
 my @pre_instances = (
   Bugzilla::Extension::MozChangeField::Pre::CanConfirm->new,
+  Bugzilla::Extension::MozChangeField::Pre::CheckinNeededTB->new,
   Bugzilla::Extension::MozChangeField::Pre::CommentClosedBugs->new,
   Bugzilla::Extension::MozChangeField::Pre::CustomField->new,
   Bugzilla::Extension::MozChangeField::Pre::Graveyard->new,
@@ -50,6 +52,15 @@ my @post_instances = (
 
 our $VERSION = '0.1';
 
+sub bug_start_of_set_all {
+  my ($self, $args) = @_;
+
+  foreach my $instance (@pre_instances) {
+    next if !$instance->can('evaluate_set_all');
+    $instance->evaluate_set_all($args);
+  }
+}
+
 sub bug_check_can_change_field {
   my ($self, $args) = @_;
 
diff --git a/extensions/MozChangeField/lib/Pre/CheckinNeededTB.pm b/extensions/MozChangeField/lib/Pre/CheckinNeededTB.pm
new file mode 100644
index 0000000000..bf4da6c250
--- /dev/null
+++ b/extensions/MozChangeField/lib/Pre/CheckinNeededTB.pm
@@ -0,0 +1,65 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# This Source Code Form is "Incompatible With Secondary Licenses", as
+# defined by the Mozilla Public License, v. 2.0.
+
+package Bugzilla::Extension::MozChangeField::Pre::CheckinNeededTB;
+
+use 5.10.1;
+
+use Moo;
+
+use Bugzilla::Error;
+
+use constant CHECKIN_KEYWORD => 'checkin-needed-tb';
+use constant PRODUCTS        => ('Calendar', 'MailNews Core', 'Thunderbird');
+
+sub evaluate_set_all {
+  my ($self, $args) = @_;
+
+  my $bug    = $args->{bug};
+  my $params = $args->{params};
+
+  my $product
+    = exists $params->{product} ? $params->{product} : $bug->product_obj->name;
+  return unless grep { $_ eq $product } PRODUCTS;
+
+  my $keyword_params = $params->{keywords};
+  return unless $keyword_params;
+
+  # This rule only applies when checkin-needed-tb is newly added.
+  return if $bug->has_keyword(CHECKIN_KEYWORD);
+
+  my $keyword_added;
+  if (exists $keyword_params->{set}) {
+    $keyword_added = _contains_checkin_keyword($keyword_params->{set});
+  }
+  else {
+    $keyword_added
+      = _contains_checkin_keyword($keyword_params->{add})
+      && !_contains_checkin_keyword($keyword_params->{remove});
+  }
+  return unless $keyword_added;
+
+  my $milestone = exists $params->{target_milestone}
+    ? $params->{target_milestone}
+    : $bug->target_milestone;
+
+  if (!defined $milestone || $milestone eq '' || $milestone eq '---') {
+    ThrowUserError('mozchangefield_checkin_needed_tb_requires_milestone');
+  }
+}
+
+sub _contains_checkin_keyword {
+  my ($keywords) = @_;
+  return 0 unless defined $keywords;
+
+  my @keywords
+    = ref $keywords eq 'ARRAY' ? @$keywords : split(/[\s,]+/, $keywords);
+
+  return scalar grep { lc($_) eq CHECKIN_KEYWORD } @keywords;
+}
+
+1;
diff --git a/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl b/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl
index 62ca30ec20..33019aa691 100644
--- a/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl
+++ b/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl
@@ -14,4 +14,9 @@
   [% title = "Invalid Change" %]
   You need "editbugs" permissions to alter the '[% field FILTER html %]' field.
 
+[% ELSIF error == "mozchangefield_checkin_needed_tb_requires_milestone" %]
+  [% title = "Target Milestone Required" %]
+  A target milestone must be selected when adding the
+  <b>checkin-needed-tb</b> keyword.
+
 [% END %]
diff --git a/t/bmo/checkin-needed-tb-milestone.t b/t/bmo/checkin-needed-tb-milestone.t
new file mode 100755
index 0000000000..ad11f883cf
--- /dev/null
+++ b/t/bmo/checkin-needed-tb-milestone.t
@@ -0,0 +1,144 @@
+#!/usr/bin/env perl
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# This Source Code Form is "Incompatible With Secondary Licenses", as
+# defined by the Mozilla Public License, v. 2.0.
+
+use 5.10.1;
+
+use strict;
+use warnings;
+
+use lib qw(. lib local/lib/perl5);
+
+use Test::More;
+
+require './extensions/MozChangeField/lib/Pre/CheckinNeededTB.pm';
+
+{
+  package Local::Product;
+
+  sub new {
+    my ($class, $name) = @_;
+    return bless {name => $name}, $class;
+  }
+
+  sub name {
+    return $_[0]->{name};
+  }
+}
+
+{
+  package Local::Bug;
+
+  sub new {
+    my ($class, %args) = @_;
+    return bless \%args, $class;
+  }
+
+  sub product_obj {
+    return Local::Product->new($_[0]->{product});
+  }
+
+  sub has_keyword {
+    return $_[0]->{has_keyword};
+  }
+
+  sub target_milestone {
+    return $_[0]->{milestone};
+  }
+}
+
+package main;
+
+my $rule
+  = Bugzilla::Extension::MozChangeField::Pre::CheckinNeededTB->new;
+
+sub evaluate_rule {
+  my (%args) = @_;
+
+  my $bug = Local::Bug->new(
+    product     => $args{product} || 'Thunderbird',
+    has_keyword => $args{has_keyword} || 0,
+    milestone   => exists $args{milestone} ? $args{milestone} : '---',
+  );
+
+  eval {
+    $rule->evaluate_set_all({
+      bug    => $bug,
+      params => $args{params},
+    });
+  };
+
+  return $@;
+}
+
+{
+  no warnings qw(once redefine);
+
+  local
+    *Bugzilla::Extension::MozChangeField::Pre::CheckinNeededTB::ThrowUserError
+    = sub { die "$_[0]\n"; };
+
+  foreach my $product ('Calendar', 'MailNews Core', 'Thunderbird') {
+    like(
+      evaluate_rule(
+        product => $product,
+        params  => {keywords => {add => ['checkin-needed-tb']}},
+      ),
+      qr/mozchangefield_checkin_needed_tb_requires_milestone/,
+      "Adding checkin-needed-tb without a milestone is rejected for $product"
+    );
+  }
+
+  is(
+    evaluate_rule(
+      milestone => 'Thunderbird 153',
+      params    => {keywords => {add => ['checkin-needed-tb']}},
+    ),
+    '',
+    'An existing target milestone allows the keyword'
+  );
+
+  is(
+    evaluate_rule(
+      params => {
+        keywords         => {add => ['checkin-needed-tb']},
+        target_milestone => 'Thunderbird 153',
+      },
+    ),
+    '',
+    'The keyword and target milestone can be set together'
+  );
+
+  is(
+    evaluate_rule(
+      product => 'Firefox',
+      params  => {keywords => {add => ['checkin-needed-tb']}},
+    ),
+    '',
+    'Other products are unaffected'
+  );
+
+  is(
+    evaluate_rule(
+      has_keyword => 1,
+      params      => {keywords => {add => ['another-keyword']}},
+    ),
+    '',
+    'A bug that already has checkin-needed-tb is unaffected'
+  );
+
+  like(
+    evaluate_rule(
+      params => {keywords => {set => ['checkin-needed-tb']}},
+    ),
+    qr/mozchangefield_checkin_needed_tb_requires_milestone/,
+    'Setting the complete keyword list is also validated'
+  );
+}
+
+done_testing();