diff --git a/sys/teken/sequences b/sys/teken/sequences
index 8f7bfb443881..957665a02f53 100644
--- a/sys/teken/sequences
+++ b/sys/teken/sequences
@@ -104,6 +104,7 @@ C25ADBG	Cons25 set adapter background		^[ [ = G	r
 C25ADFG	Cons25 set adapter foreground		^[ [ = F	r
 C25BLPD	Cons25 set bell pitch duration		^[ [ = B	r r
 C25CURS	Cons25 set cursor type			^[ [ = S	r
+C25MODE	Cons25 set terminal mode		^[ [ = T	r
 C25VTSW	Cons25 switch virtual terminal		^[ [ z		r
 
 # VT52 compatibility
diff --git a/sys/teken/teken_subr_compat.h b/sys/teken/teken_subr_compat.h
index c642af71770b..088f378af588 100644
--- a/sys/teken/teken_subr_compat.h
+++ b/sys/teken/teken_subr_compat.h
@@ -87,6 +87,20 @@ teken_subr_cons25_set_bell_pitch_duration(teken_t *t, unsigned int pitch,
 	    (duration & 0xffff));
 }
 
+static void
+teken_subr_cons25_set_terminal_mode(teken_t *t, unsigned int mode)
+{
+
+	switch (mode) {
+	case 0:	/* Switch terminal to xterm. */
+		t->t_stateflags &= ~TS_CONS25;
+		break;
+	case 1: /* Switch terminal to cons25. */
+		t->t_stateflags |= TS_CONS25;
+		break;
+	}
+}
+
 #if 0
 static void
 teken_subr_vt52_decid(teken_t *t)