A pivotal surfboard fin assembly for use on a surfboard, the assembly comprising: an insert bracket having a board face mountable to the bottom surface of the surfboard and a bracket face with a lock pin aperture and a flex plug cavity; an elastomeric flex plug having a flex plug hole, a first flex plug surface, and a second flex plug surface, the flex plug disposed in the flex plug cavity; a surfboard fin having a fin blade, a mounting edge, a flex pin channel, a fixed lock pin mountable to the surfboard fin and disposable into the lock pin aperture, a biased flex pin slidably disposable into the flex pin channel and the flex plug hole; and a first compression pin and a second compression pin engaged with the insert bracket and operative to laterally compress the flex plug.