diff --git a/putty.h b/putty.h
index 0ee3a7b6..e413552c 100644
--- a/putty.h
+++ b/putty.h
@@ -363,7 +363,8 @@ typedef enum {
     MBT_NOTHING,
     MBT_LEFT, MBT_MIDDLE, MBT_RIGHT,   /* `raw' button designations */
     MBT_SELECT, MBT_EXTEND, MBT_PASTE, /* `cooked' button designations */
-    MBT_WHEEL_UP, MBT_WHEEL_DOWN       /* mouse wheel */
+    MBT_WHEEL_UP, MBT_WHEEL_DOWN,      /* vertical mouse wheel */
+    MBT_WHEEL_LEFT, MBT_WHEEL_RIGHT    /* horizontal mouse wheel */
 } Mouse_Button;
 
 typedef enum {
diff --git a/terminal/terminal.c b/terminal/terminal.c
index e4063b66..34a8b2f2 100644
--- a/terminal/terminal.c
+++ b/terminal/terminal.c
@@ -7223,6 +7223,14 @@ void term_mouse(Terminal *term, Mouse_Button braw, Mouse_Button bcooked,
                 encstate = 0x41;
                 wheel = true;
                 break;
+              case MBT_WHEEL_LEFT:
+                encstate = 0x42;
+                wheel = true;
+                break;
+              case MBT_WHEEL_RIGHT:
+                encstate = 0x43;
+                wheel = true;
+                break;
               case MBT_NOTHING:
                 assert( a == MA_MOVE );
                 encstate = 0x03; // release; no buttons pressed
diff --git a/unix/window.c b/unix/window.c
index dbb0ec69..934311f2 100644
--- a/unix/window.c
+++ b/unix/window.c
@@ -169,7 +169,7 @@ struct GtkFrontend {
     guint32 input_event_time; /* Timestamp of the most recent input event. */
     GtkWidget *dialogs[DIALOG_SLOT_LIMIT];
 #if GTK_CHECK_VERSION(3,4,0)
-    gdouble cumulative_scroll;
+    gdouble cumulative_hscroll, cumulative_vscroll;
 #endif
     /* Cached things out of conf that we refer to a lot */
     int bold_style;
@@ -2111,8 +2111,8 @@ void input_method_commit_event(GtkIMContext *imc, gchar *str, gpointer data)
 #define SCROLL_INCREMENT_LINES 5
 
 #if GTK_CHECK_VERSION(3,4,0)
-gboolean scroll_internal(GtkFrontend *inst, gdouble delta, guint state,
-                         gdouble ex, gdouble ey)
+gboolean scroll_internal(GtkFrontend *inst, gdouble xdelta, gdouble ydelta,
+                         guint state, gdouble ex, gdouble ey)
 {
     int x, y;
     bool shift, ctrl, alt, raw_mouse_mode;
@@ -2130,22 +2130,22 @@ gboolean scroll_internal(GtkFrontend *inst, gdouble delta, guint state,
                       !(shift && conf_get_bool(inst->conf,
                                                CONF_mouse_override)));
 
-    inst->cumulative_scroll += delta * SCROLL_INCREMENT_LINES;
+    inst->cumulative_vscroll += ydelta * SCROLL_INCREMENT_LINES;
 
     if (!raw_mouse_mode) {
-        int scroll_lines = (int)inst->cumulative_scroll; /* rounds toward 0 */
+        int scroll_lines = (int)inst->cumulative_vscroll; /* rounds toward 0 */
         if (scroll_lines) {
             term_scroll(inst->term, 0, scroll_lines);
-            inst->cumulative_scroll -= scroll_lines;
+            inst->cumulative_vscroll -= scroll_lines;
         }
         return true;
     } else {
-        int scroll_events = (int)(inst->cumulative_scroll /
+        int scroll_events = (int)(inst->cumulative_vscroll /
                                   SCROLL_INCREMENT_LINES);
         if (scroll_events) {
             int button;
 
-            inst->cumulative_scroll -= scroll_events * SCROLL_INCREMENT_LINES;
+            inst->cumulative_vscroll -= scroll_events * SCROLL_INCREMENT_LINES;
 
             if (scroll_events > 0) {
                 button = MBT_WHEEL_DOWN;
@@ -2159,6 +2159,35 @@ gboolean scroll_internal(GtkFrontend *inst, gdouble delta, guint state,
                            MA_CLICK, x, y, shift, ctrl, alt);
             }
         }
+
+        /*
+         * Now do the same for horizontal scrolling. But because we
+         * _only_ use that for passing through to mouse reporting, we
+         * don't even collect the scroll deltas while not in
+         * raw_mouse_mode. (Otherwise there would likely be a huge
+         * unexpected lurch when raw_mouse_mode was enabled!)
+         */
+        inst->cumulative_hscroll += xdelta * SCROLL_INCREMENT_LINES;
+        scroll_events = (int)(inst->cumulative_hscroll /
+                              SCROLL_INCREMENT_LINES);
+        if (scroll_events) {
+            int button;
+
+            inst->cumulative_hscroll -= scroll_events * SCROLL_INCREMENT_LINES;
+
+            if (scroll_events > 0) {
+                button = MBT_WHEEL_RIGHT;
+            } else {
+                button = MBT_WHEEL_LEFT;
+                scroll_events = -scroll_events;
+            }
+
+            while (scroll_events-- > 0) {
+                term_mouse(inst->term, button, translate_button(button),
+                           MA_CLICK, x, y, shift, ctrl, alt);
+            }
+        }
+
         return true;
     }
 }
@@ -2260,7 +2289,7 @@ gboolean scroll_event(GtkWidget *widget, GdkEventScroll *event, gpointer data)
 #if GTK_CHECK_VERSION(3,4,0)
     gdouble dx, dy;
     if (gdk_event_get_scroll_deltas((GdkEvent *)event, &dx, &dy)) {
-        return scroll_internal(inst, dy, event->state, event->x, event->y);
+        return scroll_internal(inst, dx, dy, event->state, event->x, event->y);
     } else if (!gdk_event_get_scroll_direction((GdkEvent *)event, &dir)) {
         return false;
     }
@@ -5276,7 +5305,8 @@ void new_session_window(Conf *conf, const char *geometry_string)
     inst->wintitle = inst->icontitle = NULL;
     inst->drawtype = DRAWTYPE_DEFAULT;
 #if GTK_CHECK_VERSION(3,4,0)
-    inst->cumulative_scroll = 0.0;
+    inst->cumulative_vscroll = 0.0;
+    inst->cumulative_hscroll = 0.0;
 #endif
     inst->drawing_area_setup_needed = true;
 
diff --git a/windows/window.c b/windows/window.c
index 63cc58ed..e59bebce 100644
--- a/windows/window.c
+++ b/windows/window.c
@@ -74,6 +74,9 @@
 #ifndef WM_MOUSEWHEEL
 #define WM_MOUSEWHEEL 0x020A           /* not defined in earlier SDKs */
 #endif
+#ifndef WM_MOUSEHWHEEL
+#define WM_MOUSEHWHEEL 0x020E          /* not defined in earlier SDKs */
+#endif
 #ifndef WHEEL_DELTA
 #define WHEEL_DELTA 120
 #endif
@@ -3403,10 +3406,11 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
         process_clipdata((HGLOBAL)lParam, wParam);
         return 0;
       default:
-        if (message == wm_mousewheel || message == WM_MOUSEWHEEL) {
+        if (message == wm_mousewheel || message == WM_MOUSEWHEEL
+                                                || message == WM_MOUSEHWHEEL) {
             bool shift_pressed = false, control_pressed = false;
 
-            if (message == WM_MOUSEWHEEL) {
+            if (message == WM_MOUSEWHEEL || message == WM_MOUSEHWHEEL) {
                 wheel_accumulator += (short)HIWORD(wParam);
                 shift_pressed=LOWORD(wParam) & MK_SHIFT;
                 control_pressed=LOWORD(wParam) & MK_CONTROL;
@@ -3425,10 +3429,10 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
 
                 /* reduce amount for next time */
                 if (wheel_accumulator > 0) {
-                    b = MBT_WHEEL_UP;
+                    b = message == WM_MOUSEHWHEEL ? MBT_WHEEL_RIGHT : MBT_WHEEL_UP;
                     wheel_accumulator -= WHEEL_DELTA;
                 } else if (wheel_accumulator < 0) {
-                    b = MBT_WHEEL_DOWN;
+                    b =  message == WM_MOUSEHWHEEL ? MBT_WHEEL_LEFT : MBT_WHEEL_DOWN;
                     wheel_accumulator += WHEEL_DELTA;
                 } else
                     break;
@@ -3448,7 +3452,7 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
                                    TO_CHR_Y(p.y), shift_pressed,
                                    control_pressed, is_alt_pressed());
                     } /* else: not sure when this can fail */
-                } else {
+                } else if (message != WM_MOUSEHWHEEL) {
                     /* trigger a scroll */
                     term_scroll(term, 0,
                                 b == MBT_WHEEL_UP ?