Abstract: A folding swim fin which comprises a base, arms, an elastic membrane tensioned between the arms and elements for attaching the fin to the swimmer's feet. Two opposite edges (1 and 2) of the flexible membrane (3) are attached each to one elastic strip (5). The flexible membrane (3) is made of plastic material, preferably reinforced with fibres. A preferred variant of the invention is a membrane woven with durable fibres. The strip (5) is attached to a rigid beam (6) of the base, belonging to a pair of beams connected to each other by a hinge (7) placed at a distance of 0 to ? of the beam (6) length from the beam end opposite to the place of the strip (5) attachment.