Adjust Opacity of Multiple Layers in GIMP

Adjusting the opacity of multiple layers simultaneously in modern GIMP releases can be accomplished either through native multi-layer selection in newer versions (GIMP 2.99 and 3.0+) or by utilizing Layer Groups in standard stable versions like GIMP 2.10. Both methods allow you to modify transparency across multiple elements uniformly without altering each layer individually.

Method 1: Using Multi-Layer Selection (GIMP 2.99 / GIMP 3.0+)

Recent versions of GIMP introduce true multi-layer selection directly inside the Layers dockable dialog:

1. Select Multiple Layers: Open the Layers panel. Hold down Ctrl (Windows/Linux) or Cmd (macOS) and click each layer you want to modify. To select a continuous range of layers, click the first layer, hold Shift, and click the last layer.
2. Change the Opacity: Locate the Opacity slider at the top of the Layers panel.
3. Apply the Value: Drag the slider or type a numeric percentage and press Enter. The opacity adjustment applies simultaneously to every selected layer.

Method 2: Using Layer Groups (GIMP 2.10 and Newer)

If you are using GIMP 2.10, which does not support simultaneous multi-layer property adjustments natively, Layer Groups serve as the most efficient solution:

1. Create a Layer Group: Click the Create a new layer group icon (represented by a folder) at the bottom of the Layers dialog, or right-click within the Layers panel and select New Layer Group.
2. Move Layers into the Group: Drag and drop all the layers you wish to adjust into the newly created folder.
3. Adjust Group Opacity: Click on the Layer Group itself to select it.
4. Move the Slider: Adjust the Opacity slider at the top of the panel. Lowering the group's opacity uniformly reduces the opacity of all enclosed layers while preserving their relative transparency levels to each other.