An adjustable fin for use on a surfboard, the adjustable fin comprising: a base comprising: at least one mounting means for mounting the adjustable fin to a surfboard; and the base comprising an adjustment member extending in a direction contrary to the mounting means; a fin section comprising two outer fin surfaces which meet at a leading edge and a trailing edge; the fin section comprising an underside surface comprising an opening to an internal cavity within the fin section, the internal cavity configured to house the adjustment member of the base and enable slidable movement of the adjustment member in a direction towards the leading edge or the trailing edge; wherein the adjustable fin further comprises a locking means that can be manipulated from an outer fin surface and which is accommodated in a hole from the outer fin surface through into the internal cavity, wherein the adjustment member comprises three or more teeth, and a valley between two teeth form a locking position, wherein the locking means is preferably configured to releasably couple to the adjustment member at one of two or more locking positions thereby preventing slidable movement of the adjustment member, wherein the locking means comprises at least one knob which is accessible for a user at an outer fin surface of the fin section for manipulating the locking means, and wherein turning the knob uncouples the locking means from the adjustment member, or recouples the locking means to the adjustment member, wherein the fin section can be adjusted relative to the base by manipulating the locking means by turning the knob to uncouple the locking means from the adjustment member at a first locking position, slidably moving the adjustment member through the internal cavity, and turning the knob to releasably couple the locking means to the adjustment member at a second locking position. a base comprising: at least one mounting means for mounting the adjustable fin to a surfboard; and the base comprising an adjustment member extending in a direction contrary to the mounting means; at least one mounting means for mounting the adjustable fin to a surfboard; and the base comprising an adjustment member extending in a direction contrary to the mounting means; a fin section comprising two outer fin surfaces which meet at a leading edge and a trailing edge; the fin section comprising an underside surface comprising an opening to an internal cavity within the fin section, the internal cavity configured to house the adjustment member of the base and enable slidable movement of the adjustment member in a direction towards the leading edge or the trailing edge; wherein the adjustable fin further comprises a locking means that can be manipulated from an outer fin surface and which is accommodated in a hole from the outer fin surface through into the internal cavity, wherein the adjustment member comprises three or more teeth, and a valley between two teeth form a locking position, wherein the locking means is preferably configured to releasably couple to the adjustment member at one of two or more locking positions thereby preventing slidable movement of the adjustment member, wherein the locking means comprises at least one knob which is accessible for a user at an outer fin surface of the fin section for manipulating the locking means, and wherein turning the knob uncouples the locking means from the adjustment member, or recouples the locking means to the adjustment member, wherein the fin section can be adjusted relative to the base by manipulating the locking means by turning the knob to uncouple the locking means from the adjustment member at a first locking position, slidably moving the adjustment member through the internal cavity, and turning the knob to releasably couple the locking means to the adjustment member at a second locking position.