diff --git a/include/sway/config.h b/include/sway/config.h
index 632027ea..3dedbec8 100644
--- a/include/sway/config.h
+++ b/include/sway/config.h
@@ -198,6 +198,7 @@ struct output_config {
 	int enabled;
 	int width, height;
 	float refresh_rate;
+	int custom_mode;
 	int x, y;
 	float scale;
 	int32_t transform;
diff --git a/sway/commands/output/mode.c b/sway/commands/output/mode.c
index bcfce372..5b710713 100644
--- a/sway/commands/output/mode.c
+++ b/sway/commands/output/mode.c
@@ -12,6 +12,14 @@ struct cmd_results *output_cmd_mode(int argc, char **argv) {
 
 	struct output_config *output = config->handler_context.output_config;
 
+	if (strcmp(argv[0], "--custom") == 0) {
+		argv++;
+		argc--;
+		output->custom_mode = 1;
+	} else {
+		output->custom_mode = 0;
+	}
+
 	char *end;
 	output->width = strtol(*argv, &end, 10);
 	if (*end) {
diff --git a/sway/config/output.c b/sway/config/output.c
index 42deec67..1d21e52f 100644
--- a/sway/config/output.c
+++ b/sway/config/output.c
@@ -42,6 +42,7 @@ struct output_config *new_output_config(const char *name) {
 	oc->enabled = -1;
 	oc->width = oc->height = -1;
 	oc->refresh_rate = -1;
+	oc->custom_mode = -1;
 	oc->x = oc->y = -1;
 	oc->scale = -1;
 	oc->transform = -1;
@@ -74,6 +75,9 @@ void merge_output_config(struct output_config *dst, struct output_config *src) {
 	if (src->refresh_rate != -1) {
 		dst->refresh_rate = src->refresh_rate;
 	}
+	if (src->custom_mode != -1) {
+		dst->custom_mode = src->custom_mode;
+	}
 	if (src->transform != -1) {
 		dst->transform = src->transform;
 	}
@@ -202,11 +206,13 @@ struct output_config *store_output_config(struct output_config *oc) {
 }
 
 static bool set_mode(struct wlr_output *output, int width, int height,
-		float refresh_rate) {
+		float refresh_rate, bool custom) {
 	int mhz = (int)(refresh_rate * 1000);
-	if (wl_list_empty(&output->modes)) {
+
+	if (wl_list_empty(&output->modes) || custom) {
 		sway_log(SWAY_DEBUG, "Assigning custom mode to %s", output->name);
-		return wlr_output_set_custom_mode(output, width, height, mhz);
+		return wlr_output_set_custom_mode(output, width, height,
+			refresh_rate > 0 ? mhz : 0);
 	}
 
 	struct wlr_output_mode *mode, *best = NULL;
@@ -261,8 +267,8 @@ bool apply_output_config(struct output_config *oc, struct sway_output *output) {
 	if (oc && oc->width > 0 && oc->height > 0) {
 		sway_log(SWAY_DEBUG, "Set %s mode to %dx%d (%f Hz)", oc->name, oc->width,
 			oc->height, oc->refresh_rate);
-		modeset_success =
-			set_mode(wlr_output, oc->width, oc->height, oc->refresh_rate);
+		modeset_success = set_mode(wlr_output, oc->width, oc->height,
+			oc->refresh_rate, oc->custom_mode == 1);
 	} else if (!wl_list_empty(&wlr_output->modes)) {
 		struct wlr_output_mode *mode =
 			wl_container_of(wlr_output->modes.prev, mode, link);
diff --git a/sway/sway-output.5.scd b/sway/sway-output.5.scd
index da17e1c1..ae6ced24 100644
--- a/sway/sway-output.5.scd
+++ b/sway/sway-output.5.scd
@@ -24,12 +24,16 @@ must be separated by one space. For example:
 
 # COMMANDS
 
-*output* <name> mode|resolution|res <WIDTHxHEIGHT>[@<RATE>[Hz]]
+*output* <name> mode|resolution|res [--custom] <WIDTHxHEIGHT>[@<RATE>[Hz]]
 	Configures the specified output to use the given mode. Modes are a
 	combination of width and height (in pixels) and a refresh rate that your
 	display can be configured to use. For a list of available modes for each
 	output, use *swaymsg -t get_outputs*.
 
+	To set a custom mode not listed in the list of available modes, use
+	*--custom*. You should probably only use this if you know what you're
+	doing.
+
 	Examples:
 
 		output HDMI-A-1 mode 1920x1080