Abstract: A retainer is described for holding the fin box of a surfboard, windsurfer, or small sailboat very securely to the rest of the board to greatly resist breakout of the fin box from the board, which is of relatively low cost and of light weight. The retainer is formed primarily of foam of a density greater than the foam in the rest of the board, and is molded to the fin fox to bond thereto. The retainer extends up from the fin box so the assembly formed by the retainer and fin box has a height equal to the thickness of the unfinished board. A groove is cut in the unfinished board through the entire thickness of the board, and the retainer/fin box assembly is placed in the groove with the upper and lower surfaces of the assembly even with the upper and lower surfaces of the unfinished board.