Abstract: A fin box adapter for securing narrow based fins in cavity-type sailboard fin boxes comprising a generally trapezoidal block member having an outer surface configuration corresponding to an inner cavity surface configuration of a cavity fin box, and an inner slot-like cavity for receiving a bottom portion of a modified narrow fin base. In use, a narrow based fin is modified by trimming down any excessive length to fit the inner slot-like cavity and one or more press-fit cross-pins are used to secure the fin base within the inner slot-like cavity of the fin box adapter. A template is used to assist the installer in trimming down the fin base. The top surface of the fin box adapter is provided with one or more threaded holes which cooperate with the existing mounting hardware of the cavity fin box of the sailboard. In an alternate embodiment, a single threaded press-fitted cylindrical block is used in favor of the pins to secure the fin within the slot-like cavity.