A surfboard fin is a foil (see Foil (fluid mechanics) ), specifically, a hydrofoil that is mounted near the tail of a surfboard such that it is more or less perpendicular to the bottom of the board. The fin provides lateral lift (See Lift (force))opposed to oncoming water as a surfer rides a shoaling ocean wave, and also stabilizes the surfboard's trajectory, and as a result of the effects of the foil, fins are used (along with the bottom of the surfboard) as control surfaces by the rider. Surfboard fins were thus an important advancement in the maneuvering performance of surfboards. Surfboard fins may be arrayed in various number and configuration, and many different shapes, sizes, and materials are and have been made and used.

Surfboard Fins were invented in Hawaii, along with the surfboard itself. They are now more or less universally used on surfboards.

Glass on fins are fins that are permanently connected to the surfboard through fiberglass. This type of fin was mainly used on older model surfboards. Glass on fins are broken easily and are hard to repair. You rarely see these types of fins today because a different type of fin has replaced them.