Abstract: A t-shaped, hydrodynamic fin (“t-fin”) for surfboards designed to maximize performance and maneuverability of the surfboard by creating, in conjunction with two vertical side fins, two chambers to funnel and control the flow of water. The crosspiece of the t-fin is parallel to the surfboard and is horizontal when the surfboard is level from side to side. But also functions as a vertical fin when the board is tilted sideways (in either direction) and the side fins lose their verticality. The t-fin enhances surfboard maneuverability by maintaining fin perpendicularity on the board at all times. The t-fin is fastened to the underside of the surfboard, at the stern, but a second t-fin may be fastened at the same time to the bow underside, to achieve a surfboard that can be used reversibly. T-fins are custom-made, shaped and contoured, and the crosspiece of the t-fin affixed, to achieve the desired hydrodynamic effect.