The gill membrane is the film that surrounds the outer edge of the gill cover and tightly covers the gill slit during water ingestion (the moment of inhalation). Gill membranes can be fused with each other or attached to the intergill space.

The membrane formed by the ectoderm of the bottom of the gill sulcus and the entoderm of the bottom of the pharyngeal pocket. It encircles the gill lids from behind and serves to close the gill openings more tightly.

The gill membranes can be attached to the intergill space (in carps) or loose (in herring fishes). In some fish, the gill membranes are fused together to form a fold (beluga).