diff --git a/layout-custom.c b/layout-custom.c
index 703dd55ef..7c9c68bdc 100644
--- a/layout-custom.c
+++ b/layout-custom.c
@@ -61,6 +61,7 @@ static int			 layout_append_geometry(char *, size_t, u_int,
 static int			 layout_check(struct layout_cell *);
 static int			 layout_check_geometry(u_int, u_int, int, int);
 static int			 layout_parse_char(struct layout_parse_ctx *, char);
+static int			 layout_parse_version(struct layout_parse_ctx *);
 static int			 layout_construct(struct layout_parse_ctx *,
 				     struct layout_cell *, struct layout_cell **);
 static void			 layout_assign(struct window *, struct window_pane **,
@@ -137,7 +138,7 @@ layout_dump(struct window *w, struct layout_cell *root)
 {
 	char	 layout[LAYOUT_STRING_MAX], *out;
 
-	*layout = '\0';
+	strlcpy(layout, "v2:", sizeof layout);
 	if (layout_append(w, root, layout, sizeof layout) != 0)
 		return (NULL);
 
@@ -585,6 +586,21 @@ layout_parse_checksum(struct layout_parse_ctx *ctx, u_short *csum)
 	return (1);
 }
 
+/* Parse an optional outer layout version. */
+static int
+layout_parse_version(struct layout_parse_ctx *ctx)
+{
+	layout_skip_space(ctx);
+	if (ctx->ptr == ctx->end || *ctx->ptr != 'v')
+		return (0);
+	if ((size_t)(ctx->end - ctx->ptr) < 3 || ctx->ptr[1] != '2' ||
+	    ctx->ptr[2] != ':')
+		return (-1);
+	ctx->ptr += 3;
+	layout_skip_space(ctx);
+	return (1);
+}
+
 /* Construct one cell, accepting an optional checksum before nested cells. */
 static int
 layout_construct(struct layout_parse_ctx *ctx, struct layout_cell *parent,
@@ -900,7 +916,7 @@ layout_prepare(struct window *w, const char *layout, char **cause)
 	const char		*body;
 	u_int			 npanes, ncells, old_ncells;
 	u_short			 csum;
-	int			 has_checksum, pane_ids = 0;
+	int			 has_checksum, pane_ids = 0, version;
 
 	if (strlen(layout) >= LAYOUT_STRING_MAX) {
 		*cause = xstrdup("invalid layout");
@@ -917,6 +933,11 @@ layout_prepare(struct window *w, const char *layout, char **cause)
 		*cause = xstrdup("invalid layout checksum");
 		return (NULL);
 	}
+	version = layout_parse_version(&ctx);
+	if (version == -1) {
+		*cause = xstrdup("invalid layout version");
+		return (NULL);
+	}
 	if (layout_construct(&ctx, NULL, &root) != 0) {
 		*cause = xstrdup("invalid layout");
 		layout_free_cell(root, 0);
@@ -928,6 +949,11 @@ layout_prepare(struct window *w, const char *layout, char **cause)
 		layout_free_cell(root, 0);
 		return (NULL);
 	}
+	if (version == 1 && ctx.format != LAYOUT_FORMAT) {
+		*cause = xstrdup("invalid layout version");
+		layout_free_cell(root, 0);
+		return (NULL);
+	}
 	if (layout_resolve_relative(root,
 	    root->type == LAYOUT_WINDOWPANE ? w->sx : root->sx,
 	    root->type == LAYOUT_WINDOWPANE ? w->sy : root->sy) != 0) {
diff --git a/regress/control-client-layout.sh b/regress/control-client-layout.sh
index f0cd72efc..f4d8460d4 100644
--- a/regress/control-client-layout.sh
+++ b/regress/control-client-layout.sh
@@ -38,11 +38,12 @@ sleep 1
 # Default control output is legacy; window-layout-new output is current.
 awk '/^layout:/ { print }' "$OUT1" | while IFS= read -r line; do
 	case "$line" in
-	*window-layout-new*|*%0,*|*';'*) exit 1 ;;
+	*window-layout-new*|*v2:*|*%0,*|*';'*) exit 1 ;;
 	esac
 done || exit 1
 [ "$(grep -ac '^layout:' "$OUT1")" -eq 6 ] || exit 1
-grep -a '^layout:.*window-layout-new.*%0,' "$OUT2" >/dev/null || exit 1
+grep -a '^layout:.*window-layout-new.*,v2:.*%0,' "$OUT2" >/dev/null ||
+    exit 1
 
 # Toggling the flag changes subsequent format expansion for the same client.
 printf '%s\n' "refresh-client -f window-layout-new" >&3
@@ -50,7 +51,8 @@ printf '%s\n' "display-message -p 'new:#{client_flags}:#{window_layout}'" >&3
 printf '%s\n' "refresh-client -f '!window-layout-new'" >&3
 printf '%s\n' "display-message -p 'legacy:#{client_flags}:#{window_layout}'" >&3
 sleep 1
-grep -a '^new:.*window-layout-new.*%0,' "$OUT1" >/dev/null || exit 1
+grep -a '^new:.*window-layout-new.*,v2:.*%0,' "$OUT1" >/dev/null ||
+    exit 1
 grep -a '^legacy:' "$OUT1" | tail -1 | grep -v '%0,' >/dev/null || exit 1
 
 # One layout change is formatted independently for each connected client.
@@ -62,10 +64,12 @@ LEGACY=$(grep -a '^%layout-change ' "$OUT1" | tail -1)
 CURRENT=$(grep -a '^%layout-change ' "$OUT2" | tail -1)
 [ -n "$LEGACY" ] || exit 1
 [ -n "$CURRENT" ] || exit 1
-case "$LEGACY" in *%0,*|*';'*) exit 1 ;; esac
-case "$CURRENT" in *%0,*) ;; *) exit 1 ;; esac
+case "$LEGACY" in *v2:*|*%0,*|*';'*) exit 1 ;; esac
+case "$CURRENT" in *v2:*%0,*) ;; *) exit 1 ;; esac
 [ "$(printf '%s' "$CURRENT" | awk '{ print gsub(/%0,/, "") }')" -ge 2 ] ||
     exit 1
+[ "$(printf '%s' "$CURRENT" | awk '{ print gsub(/v2:/, "") }')" -ge 2 ] ||
+    exit 1
 
 exec 3>&-
 exec 4>&-
diff --git a/regress/layout-custom.sh b/regress/layout-custom.sh
index 3846dca64..a1bf6d15d 100755
--- a/regress/layout-custom.sh
+++ b/regress/layout-custom.sh
@@ -26,6 +26,21 @@ must_fail()
 	return 0
 }
 
+versioned()
+{
+	body="v2:$1"
+	checksum=$(printf %s "$body" | od -An -tu1 | awk '
+	    { for (i = 1; i <= NF; i++) {
+	        if ($i == 9 || $i == 10 || $i == 11 || $i == 12 ||
+	            $i == 13 || $i == 32)
+	            continue
+	        checksum = int(checksum / 2) + (checksum % 2) * 32768
+	        checksum = (checksum + $i) % 65536
+	    } }
+	    END { printf "%04x", checksum }')
+	printf '%s,%s' "$checksum" "$body"
+}
+
 $TMUX kill-server 2>/dev/null
 sleep 0.5
 $TMUX new-session -d -x 80 -y 24 || exit 1
@@ -36,20 +51,37 @@ legacy='89f5,80x24,0,0{39x24,0,0,0,40x24,40,0,1}'
 $TMUX select-layout "$legacy" || fail "legacy layout was rejected"
 layout=$($TMUX display-message -p '#{window_layout}')
 must_equal "$layout" \
-    '93fc,80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+    'cc3c,v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
 
 # Both current and legacy layouts may omit the outer checksum.
 $TMUX select-layout '80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}' || \
     fail "checksumless current layout was rejected"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
+$TMUX select-layout \
+    'v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}' || \
+    fail "checksumless versioned layout was rejected"
+must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
 $TMUX select-layout '80x24,0,0{39x24,0,0,0,40x24,40,0,1}' || \
     fail "checksumless legacy layout was rejected"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
 must_fail $TMUX select-layout \
     '0000,80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+must_fail $TMUX select-layout \
+    '0000,v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+# The old checksum for the unversioned cell does not validate v2:.
+must_fail $TMUX select-layout \
+    '93fc,v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+must_fail $TMUX select-layout \
+    'v1:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+must_fail $TMUX select-layout \
+    'v3:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+must_fail $TMUX select-layout \
+    'v2:80x24,0,0{39x24,0,0,0,40x24,40,0,1}'
+must_fail $TMUX select-layout \
+    '80x24+0+0{v2:%0,0:39x24+0+0;%1,1:40x24+40+0}'
 
 # Top-bottom containers are accepted in both current and legacy formats.
-topbottom='081e,80x24+0+0[%0,0:80x11+0+0;%1,1:80x12+0+12]'
+topbottom=$(versioned '80x24+0+0[%0,0:80x11+0+0;%1,1:80x12+0+12]')
 $TMUX select-layout "$topbottom" || fail "top-bottom layout was rejected"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$topbottom"
 must_equal "$($TMUX list-panes -F '#{pane_width}x#{pane_height},#{pane_left},#{pane_top}')" \
@@ -72,7 +104,7 @@ $TMUX select-layout "$layout" || fail "layout could not be restored"
 # tiled geometry.
 $TMUX resize-pane -t %0 -Z || fail "tiled zoom failed"
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
-    '8e46,80x24+0+0{%0,0:39x24+0+0:z;%1,1:40x24+40+0}'
+    "$(versioned '80x24+0+0{%0,0:39x24+0+0:z;%1,1:40x24+40+0}')"
 $TMUX resize-pane -t %0 -Z || fail "tiled unzoom failed"
 $TMUX select-layout "$layout" || fail "layout could not be restored"
 
@@ -115,7 +147,7 @@ must_fail $TMUX select-layout "$badnested"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
 
 # Hidden state is retained in the serialized cell flags.
-hidden='659f,80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0:h}'
+hidden=$(versioned '80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0:h}')
 $TMUX select-layout "$hidden" || fail "hidden layout was rejected"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$hidden"
 
@@ -228,7 +260,7 @@ $TMUX resize-pane -t %0 -Z || fail "unzoom failed"
 $TMUX kill-server 2>/dev/null
 sleep 0.5
 $TMUX new-session -d -x 80 -y 24 || exit 1
-legacy_negative='a9af,%0,0:20x8+0+0'
+legacy_negative=$(versioned '%0,0:20x8+0+0')
 $TMUX select-layout '20x8,-10,-20,0' || fail "legacy negative layout failed"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$legacy_negative"
 deep='%0,0:1x1+0+0'
@@ -262,43 +294,45 @@ $TMUX select-layout "$relative" || fail "relative geometry was rejected"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top}')" \
     '80,10'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
-    '0041,120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+10:f}'
+    "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+10:f}')"
 
 bottomright='120x40+0+0{%0,1:120x40+0+0;%1,0:30x10-0-0:f}'
 $TMUX select-layout "$bottomright" || fail "bottom-right geometry failed"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top}')" \
     '90,30'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
-    '2242,120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+90+30:f}'
+    "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+90+30:f}')"
 
-absolute='3a22,120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:f}'
-$TMUX select-layout "$absolute" || fail "absolute negative geometry failed"
+absolute_body='120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:f}'
+$TMUX select-layout "3a22,$absolute_body" || \
+    fail "absolute negative geometry failed"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top}')" \
     '-10,-20'
-must_equal "$($TMUX display-message -p '#{window_layout}')" "$absolute"
+must_equal "$($TMUX display-message -p '#{window_layout}')" \
+    "$(versioned "$absolute_body")"
 
 hiddenfloat='120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}'
 $TMUX select-layout "$hiddenfloat" || fail "hidden floating layout failed"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top},#{pane_width},#{pane_height}')" \
     '-10,-20,20,8'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
-    '9d83,120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}'
+    "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}')"
 must_fail $TMUX select-layout \
     '120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fhz}'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
-    '9d83,120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}'
+    "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}')"
 must_fail $TMUX select-layout \
     '120x40+0+0{%0,1:120x40+0+0;%1,0:10000x8-10000+0:f}'
 
 defaults='fcb6,120x40{%0,1:120x40;%1,0:30x10++80+10:f}'
 $TMUX select-layout "$defaults" || fail "default or doubled offsets failed"
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
-    '0041,120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+10:f}'
+    "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+10:f}')"
 
 oneoffset='8f93,120x40{%0,1:120x40;%1,0:30x10+80:f}'
 $TMUX select-layout "$oneoffset" || fail "single offset default failed"
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
-    '5fb9,120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+0:f}'
+    "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+0:f}')"
 
 $TMUX kill-server 2>/dev/null
 sleep 0.5
@@ -338,7 +372,7 @@ $TMUX new-pane -d -x 24 -y 8 -X 50 -Y 12 'sleep 100' || \
 
 layout=$($TMUX display-message -p '#{window_layout}')
 must_equal "$layout" \
-    'a6f2,120x40+0+0{%0,2:60x40+0+0;%1,3:59x40+61+0;%3,0:22x6+51+13:f;%2,1:28x8+11+6:f}'
+    "$(versioned '120x40+0+0{%0,2:60x40+0+0;%1,3:59x40+61+0;%3,0:22x6+51+13:f;%2,1:28x8+11+6:f}')"
 case "$layout" in
 *'<'*|*'>'*) fail "obsolete floating delimiters were emitted" ;;
 esac
diff --git a/tmux.1 b/tmux.1
index 027b3cd77..70514aed0 100644
--- a/tmux.1
+++ b/tmux.1
@@ -2684,8 +2684,8 @@ For example:
 .Bd -literal -offset indent
 $ tmux list\-windows
 0: ksh [159x48]
-    layout: 8ebd,159x48+0+0{%0,0:79x48+0+0;%1,1:79x48+80+0}
-$ tmux select\-layout \[aq]8ebd,159x48+0+0{%0,0:79x48+0+0;%1,1:79x48+80+0}\[aq]
+    layout: aadd,v2:159x48+0+0{%0,0:79x48+0+0;%1,1:79x48+80+0}
+$ tmux select\-layout \[aq]aadd,v2:159x48+0+0{%0,0:79x48+0+0;%1,1:79x48+80+0}\[aq]
 .Ed
 .Pp
 .Nm
@@ -3979,9 +3979,20 @@ spreads the current pane and any panes next to it out evenly.
 may be a serialized layout with an optional outer checksum followed by one
 root layout cell:
 .Bd -literal -offset indent
-layout = [checksum,]cell
+layout = [checksum,][v2:]cell
 .Ed
 .Pp
+The
+.Ql v2:
+prefix identifies the current format and is included in generated current
+layouts.
+It may be omitted when reading a current layout for compatibility with older
+output.
+An explicit
+.Ql v2:
+prefix may not be used with legacy cell syntax and unknown versions are
+rejected.
+.Pp
 A layout cell is either a container cell or a pane cell.
 Geometry gives a cell's size and position.
 .Nm
@@ -4043,8 +4054,9 @@ A layout cannot be applied to a target window with more panes than it has pane
 cells.
 .Pp
 If a window has only one pane, the root layout cell is itself a pane cell, so
-the pane ID, z-index, geometry and any flags immediately follow the outer
-checksum.
+the pane ID, z-index, geometry and any flags immediately follow the
+.Ql v2:
+prefix.
 The z-index is zero for the front pane and increases towards the back.
 It is present for every pane, including tiled panes.
 Z-indexes must be unique and contiguous from zero, with all floating panes
@@ -4071,7 +4083,7 @@ flags cannot be combined.
 .Pp
 For example, this layout contains two tiled panes and one floating pane:
 .Bd -literal -offset indent
-6939,120x40+0+0{%0,1:60x40+0+0;%1,2:59x40+61+0;%2,0:28x8+11+6:f}
+7749,v2:120x40+0+0{%0,1:60x40+0+0;%1,2:59x40+61+0;%2,0:28x8+11+6:f}
 .Ed
 .Pp
 The legacy format uses commas rather than semicolons to separate child cells
@@ -4090,12 +4102,14 @@ before pane IDs were added.
 .Pp
 The checksum is optional.
 If present, it is four hexadecimal digits and covers all non-whitespace text
-after its comma; a layout with an incorrect checksum is rejected.
+after its comma, including any version prefix; a layout with an incorrect
+checksum is rejected.
 Whitespace, including newlines, may appear between layout tokens and does not
 change the checksum, so a layout may be indented for readability.
 A checksum may also appear before any nested cell; nested checksums are
 accepted and checked but are not generated by
 .Ic list\-windows .
+Version prefixes are accepted only on the outer layout, not on nested cells.
 A layout must use one form consistently.
 In a legacy layout, panes are treated as tiled, visible and unzoomed.
 Outside control mode,
@@ -4120,7 +4134,7 @@ window-layouts = @window-id:layout [@window-id:layout ...]
 .Ed
 .Pp
 Whitespace, including newlines, may appear between the window records.
-Each window layout retains its own checksum.
+Each window layout retains its own checksum and version prefix.
 All records are checked before any window is changed.
 Unknown or duplicate window IDs are rejected.
 .Tg selectp