diff --git a/README.md b/README.md
index cda09ee..23b94f4 100644
--- a/README.md
+++ b/README.md
@@ -74,11 +74,12 @@ of small previews is displayed, making it easy to choose an image to open.
     -d           Scale all images to 100%, but fit large images into window
     -F           Use size-hints to make the window fixed/floating
     -f           Start in fullscreen mode
+    -G GAMMA     Set image gamma to GAMMA (-32..32)
     -g GEOMETRY  Set window position and size
                  (see section GEOMETRY SPECIFICATIONS of X(7))
     -i           Read file list from stdin
-    -n NUM       Start at picture NUM
     -N NAME      Set X window resource name to NAME
+    -n NUM       Start at picture NUM
     -o           Write list of marked files to stdout when quitting
     -q           Be quiet, disable warnings
     -r           Search given directories recursively for images
@@ -108,6 +109,7 @@ of small previews is displayed, making it easy to choose an image to open.
     D            Remove image from file list and go to next image
 
     m            Mark/unmark current image
+    M            Reverse all image marks
     N            Go [count] marked images forward
     P            Go [count] marked images backward
 
diff --git a/commands.c b/commands.c
index 7562bd1..49298bf 100644
--- a/commands.c
+++ b/commands.c
@@ -251,6 +251,17 @@ bool it_toggle_image_mark(arg_t a)
 	return true;
 }
 
+bool it_reverse_marks(arg_t a)
+{
+	int i, cnt = mode == MODE_IMAGE ? filecnt : tns.cnt;
+
+	for (i = 0; i < cnt; i++)
+		files[i].marked = !files[i].marked;
+	if (mode == MODE_THUMB)
+		tns.dirty = true;
+	return true;
+}
+
 bool it_navigate_marked(arg_t a)
 {
 	long n = (long) a;
diff --git a/commands.h b/commands.h
index 16aee07..ba40eed 100644
--- a/commands.h
+++ b/commands.h
@@ -55,6 +55,7 @@ bool it_n_or_last(arg_t);
 bool i_navigate_frame(arg_t);
 bool i_toggle_animation(arg_t);
 bool it_toggle_image_mark(arg_t);
+bool it_reverse_marks(arg_t);
 bool it_navigate_marked(arg_t);
 bool it_scroll_move(arg_t);
 bool it_scroll_screen(arg_t);
diff --git a/config.def.h b/config.def.h
index bc8e419..b1e9c02 100644
--- a/config.def.h
+++ b/config.def.h
@@ -101,6 +101,7 @@ static const keymap_t keys[] = {
 	{ true,   XK_space,         i_toggle_animation,   (arg_t) None },
 
 	{ false,  XK_m,             it_toggle_image_mark, (arg_t) None },
+	{ false,  XK_M,             it_reverse_marks,     (arg_t) None },
 	{ false,  XK_N,             it_navigate_marked,   (arg_t) +1 },
 	{ false,  XK_P,             it_navigate_marked,   (arg_t) -1 },
 
diff --git a/sxiv.1 b/sxiv.1
index 42ee155..1fca2dd 100644
--- a/sxiv.1
+++ b/sxiv.1
@@ -135,6 +135,9 @@ Remove current image from file list and go to next image.
 .B m
 Mark/unmark the current image.
 .TP
+.B M
+Reverse all image marks.
+.TP
 .B N
 Go
 .I count