Abstract: An expandable swim flipper includes a blade having at least an expandable portion thereof. The expandable portion radially expands in response to water resistance directed to a first surface of the blade. The surface area of the flipper is correspondingly increased to propel a swimmer at a faster rate through the water. The expandable portion radially contracts in response to water resistance directed to a second surface of the blade. The surface area of the flipper is correspondingly reduced, and is thereby subject to decreased water resistance. In a first preferred embodiment of the invention, the flipper blade is formed of expandable lateral members joined to a central member having a constant width. In a second, equally preferred embodiment, the flipper blade is formed of lateral members having a constant width and joined to an expandable central member. In a third, equally preferred embodiment of the invention, the flipper blade is expandable over its entire width.