diff --git a/tools/tools/git/git-arc.1 b/tools/tools/git/git-arc.1
--- a/tools/tools/git/git-arc.1
+++ b/tools/tools/git/git-arc.1
@@ -48,6 +48,7 @@
 .Op Ar commit Ns | Ns Ar commit-range
 .Nm
 .Cm update
+.Op Fl m Ar message
 .Op Ar commit Ns | Ns Ar commit-range
 .Sh DESCRIPTION
 The
@@ -105,6 +106,15 @@
 specified commits.
 Currently only the diff is updated; the review description and other
 metadata are not synchronized.
+If a message is specified with
+.Fl m ,
+that message is added as a note to the Differential Revision.
+If no message is supplied,
+the user's editor will be opened to provide an update message for
+each revision.
+If an empty message is supplied via
+.Fl m ,
+then no notes will be added when updating Differential Revisions.
 .El
 .Sh CONFIGURATION
 These are manipulated by
diff --git a/tools/tools/git/git-arc.sh b/tools/tools/git/git-arc.sh
--- a/tools/tools/git/git-arc.sh
+++ b/tools/tools/git/git-arc.sh
@@ -53,7 +53,7 @@
   list <commit>|<commit range>
   patch <diff1> [<diff2> ...]
   stage [-b branch] [<commit>|<commit range>]
-  update [<commit>|<commit range>]
+  update [-m message] [<commit>|<commit range>]
 
 Description:
   Create or manage FreeBSD Phabricator reviews based on git commits.  There
@@ -501,7 +501,20 @@
 
 gitarc__update()
 {
-    local commit commits diff
+    local commit commits diff have_msg msg
+
+    while getopts m: o; do
+        case "$o" in
+        m)
+            msg="$OPTARG"
+	    have_msg=1
+            ;;
+        *)
+            err_usage
+            ;;
+        esac
+    done
+    shift $((OPTIND-1))
 
     commits=$(build_commit_list "$@")
     for commit in ${commits}; do
@@ -514,8 +527,14 @@
         # The linter is stupid and applies patches to the working copy.
         # This would be tolerable if it didn't try to correct "misspelled" variable
         # names.
-        arc diff --allow-untracked --never-apply-patches --update "$diff" \
-            --head "$commit" "${commit}~"
+        if [ -n "$have_msg" ]; then
+                arc diff --message "$msg" --allow-untracked \
+                    --never-apply-patches --update "$diff" --head "$commit" \
+                    "${commit}~"
+        else
+                arc diff --allow-untracked --never-apply-patches \
+                    --update "$diff" --head "$commit" "${commit}~"
+        fi
     done
 }