diff --git a/Userland/Libraries/LibGUI/OpacitySlider.cpp b/Userland/Libraries/LibGUI/OpacitySlider.cpp
index a2cceaf061..299f1e2c0a 100644
--- a/Userland/Libraries/LibGUI/OpacitySlider.cpp
+++ b/Userland/Libraries/LibGUI/OpacitySlider.cpp
@@ -17,9 +17,6 @@ namespace GUI {
 OpacitySlider::OpacitySlider(Gfx::Orientation orientation)
     : AbstractSlider(orientation)
 {
-    // FIXME: Implement vertical mode.
-    VERIFY(orientation == Gfx::Orientation::Horizontal);
-
     set_min(0);
     set_max(100);
     set_value(100);
@@ -42,51 +39,92 @@ void OpacitySlider::paint_event(PaintEvent& event)
     Gfx::StylePainter::paint_transparency_grid(painter, inner_rect, palette());
 
     // Alpha gradient
-    for (int x = inner_rect.left(); x <= inner_rect.right(); ++x) {
-        float relative_offset = (float)x / (float)width();
+    for (int pos = inner_rect.first_edge_for_orientation(orientation()); pos <= inner_rect.last_edge_for_orientation(orientation()); ++pos) {
+        float relative_offset = (float)pos / (float)inner_rect.primary_size_for_orientation(orientation());
         float alpha = relative_offset * 255.0f;
-        Color color { 0, 0, 0, (u8)alpha };
-        painter.fill_rect({ x, inner_rect.y(), 1, inner_rect.height() }, color);
+        Color color { 0, 0, 0, static_cast<u8>(AK::min(alpha, UINT8_MAX)) };
+        if (orientation() == Gfx::Orientation::Horizontal) {
+            painter.fill_rect({ pos, inner_rect.top(), 1, inner_rect.height() }, color);
+        } else {
+            painter.fill_rect({ inner_rect.left(), pos, inner_rect.right(), 1 }, color);
+        }
     }
 
     constexpr int notch_size = 3;
-    int notch_y_top = inner_rect.top() + notch_size;
-    int notch_y_bottom = inner_rect.bottom() - notch_size;
-    int notch_x = inner_rect.left() + ((float)value() / (float)max() * (float)inner_rect.width());
+    if (orientation() == Gfx::Orientation::Horizontal) {
+        int notch_y_top = inner_rect.top() + notch_size;
+        int notch_y_bottom = inner_rect.bottom() - notch_size;
+        int notch_x = inner_rect.left() + ((float)value() / (float)max() * (float)inner_rect.width());
 
-    // Top notch
-    painter.set_pixel(notch_x, notch_y_top, palette().threed_shadow2());
-    for (int i = notch_size; i >= 0; --i) {
-        painter.set_pixel(notch_x - (i + 1), notch_y_top - i - 1, palette().threed_highlight());
-        for (int j = 0; j < i * 2; ++j) {
-            painter.set_pixel(notch_x - (i + 1) + j + 1, notch_y_top - i - 1, palette().button());
+        // Top notch
+        painter.set_pixel(notch_x, notch_y_top, palette().threed_shadow2());
+        for (int i = notch_size; i >= 0; --i) {
+            painter.set_pixel(notch_x - (i + 1), notch_y_top - i - 1, palette().threed_highlight());
+            for (int j = 0; j < i * 2; ++j) {
+                painter.set_pixel(notch_x - (i + 1) + j + 1, notch_y_top - i - 1, palette().button());
+            }
+            painter.set_pixel(notch_x + (i + 0), notch_y_top - i - 1, palette().threed_shadow1());
+            painter.set_pixel(notch_x + (i + 1), notch_y_top - i - 1, palette().threed_shadow2());
         }
-        painter.set_pixel(notch_x + (i + 0), notch_y_top - i - 1, palette().threed_shadow1());
-        painter.set_pixel(notch_x + (i + 1), notch_y_top - i - 1, palette().threed_shadow2());
-    }
 
-    // Bottom notch
-    painter.set_pixel(notch_x, notch_y_bottom, palette().threed_shadow2());
-    for (int i = 0; i < notch_size; ++i) {
-        painter.set_pixel(notch_x - (i + 1), notch_y_bottom + i + 1, palette().threed_highlight());
-        for (int j = 0; j < i * 2; ++j) {
-            painter.set_pixel(notch_x - (i + 1) + j + 1, notch_y_bottom + i + 1, palette().button());
+        // Bottom notch
+        painter.set_pixel(notch_x, notch_y_bottom, palette().threed_shadow2());
+        for (int i = 0; i < notch_size; ++i) {
+            painter.set_pixel(notch_x - (i + 1), notch_y_bottom + i + 1, palette().threed_highlight());
+            for (int j = 0; j < i * 2; ++j) {
+                painter.set_pixel(notch_x - (i + 1) + j + 1, notch_y_bottom + i + 1, palette().button());
+            }
+            painter.set_pixel(notch_x + (i + 0), notch_y_bottom + i + 1, palette().threed_shadow1());
+            painter.set_pixel(notch_x + (i + 1), notch_y_bottom + i + 1, palette().threed_shadow2());
         }
-        painter.set_pixel(notch_x + (i + 0), notch_y_bottom + i + 1, palette().threed_shadow1());
-        painter.set_pixel(notch_x + (i + 1), notch_y_bottom + i + 1, palette().threed_shadow2());
-    }
 
-    // Hairline
-    // NOTE: If we're in the whiter part of the gradient, the notch is painted as shadow between the notches.
-    //       If we're in the darker part, the notch is painted as highlight.
-    //       We adjust the hairline's x position so it lines up with the shadow/highlight of the notches.
-    u8 h = ((float)value() / (float)max()) * 255.0f;
-    if (h < 128)
-        painter.draw_line({ notch_x, notch_y_top }, { notch_x, notch_y_bottom }, Color(h, h, h, 255));
-    else
-        painter.draw_line({ notch_x - 1, notch_y_top }, { notch_x - 1, notch_y_bottom }, Color(h, h, h, 255));
+        // Hairline
+        // NOTE: If we're in the whiter part of the gradient, the notch is painted as shadow between the notches.
+        //       If we're in the darker part, the notch is painted as highlight.
+        //       We adjust the hairline's x position so it lines up with the shadow/highlight of the notches.
+        u8 h = ((float)value() / (float)max()) * 255.0f;
+        if (h < 128)
+            painter.draw_line({ notch_x, notch_y_top }, { notch_x, notch_y_bottom }, Color(h, h, h, 255));
+        else
+            painter.draw_line({ notch_x - 1, notch_y_top }, { notch_x - 1, notch_y_bottom }, Color(h, h, h, 255));
+    } else {
+        int notch_x_left = inner_rect.left() + notch_size;
+        int notch_x_right = inner_rect.right() - notch_size;
+        int notch_y = inner_rect.top() + ((float)value() / (float)max() * (float)inner_rect.height());
+
+        // Left notch
+        painter.set_pixel(notch_x_left, notch_y, palette().threed_shadow2());
+        for (int i = notch_size; i >= 0; --i) {
+            painter.set_pixel(notch_x_left - i - 1, notch_y - (i + 1), palette().threed_highlight());
+            for (int j = 0; j < i * 2; ++j) {
+                painter.set_pixel(notch_x_left - i - 1, notch_y - (i + 1) + j + 1, palette().button());
+            }
+            painter.set_pixel(notch_x_left - i - 1, notch_y + (i + 0), palette().threed_shadow1());
+            painter.set_pixel(notch_x_left - i - 1, notch_y + (i + 1), palette().threed_shadow2());
+        }
+
+        // Bottom notch
+        painter.set_pixel(notch_x_right, notch_y, palette().threed_shadow2());
+        for (int i = 0; i < notch_size; ++i) {
+            painter.set_pixel(notch_x_right + i + 1, notch_y - (i + 1), palette().threed_highlight());
+            for (int j = 0; j < i * 2; ++j) {
+                painter.set_pixel(notch_x_right + i + 1, notch_y - (i + 1) + j + 1, palette().button());
+            }
+            painter.set_pixel(notch_x_right + i + 1, notch_y + (i + 0), palette().threed_shadow1());
+            painter.set_pixel(notch_x_right + i + 1, notch_y + (i + 1), palette().threed_shadow2());
+        }
+
+        // Hairline
+        // NOTE: See above
+        u8 h = ((float)value() / (float)max()) * 255.0f;
+        if (h < 128)
+            painter.draw_line({ notch_x_left, notch_y }, { notch_x_right, notch_y }, Color(h, h, h, 255));
+        else
+            painter.draw_line({ notch_x_left, notch_y - 1 }, { notch_x_right, notch_y - 1 }, Color(h, h, h, 255));
+    }
 
     // Text label
+    // FIXME: better support text in vertical orientation, either by having a vertical option for draw_text, or only showing when there is enough space
     auto percent_text = DeprecatedString::formatted("{}%", (int)((float)value() / (float)max() * 100.0f));
     painter.draw_text(inner_rect.translated(1, 1), percent_text, Gfx::TextAlignment::Center, Color::Black);
     painter.draw_text(inner_rect, percent_text, Gfx::TextAlignment::Center, Color::White);
@@ -98,11 +136,14 @@ void OpacitySlider::paint_event(PaintEvent& event)
 int OpacitySlider::value_at(Gfx::IntPoint position) const
 {
     auto inner_rect = frame_inner_rect();
-    if (position.x() < inner_rect.left())
+    auto relevant_position = position.primary_offset_for_orientation(orientation()),
+         begin_position = inner_rect.first_edge_for_orientation(orientation()),
+         end_position = inner_rect.last_edge_for_orientation(orientation());
+    if (relevant_position < begin_position)
         return min();
-    if (position.x() > inner_rect.right())
+    if (relevant_position > end_position)
         return max();
-    float relative_offset = (float)(position.x() - inner_rect.x()) / (float)inner_rect.width();
+    float relative_offset = (float)(relevant_position - begin_position) / (float)inner_rect.primary_size_for_orientation(orientation());
 
     int range = max() - min();
     return min() + (int)(relative_offset * (float)range);
diff --git a/Userland/Libraries/LibGUI/OpacitySlider.h b/Userland/Libraries/LibGUI/OpacitySlider.h
index 92b3709091..d35f7e5af7 100644
--- a/Userland/Libraries/LibGUI/OpacitySlider.h
+++ b/Userland/Libraries/LibGUI/OpacitySlider.h
@@ -29,6 +29,9 @@ private:
 
     Gfx::IntRect frame_inner_rect() const;
 
+    virtual Optional<UISize> calculated_min_size() const override;
+    virtual Optional<UISize> calculated_preferred_size() const override;
+
     int value_at(Gfx::IntPoint) const;
 
     bool m_dragging { false };