A steering wheel for a boat rudder, which can be mounted on a rudder actuating shaft, comprising a substantially circular body (2) with an external peripheral edge (3) for gripping by a user and a central hub (4) defining an axis of rotation (L), characterized in that the body (2) is formed by a plurality of circle portions (5) which are movable with respect to each other, so as to pass selectively from an open operative configuration, where the circle portions (5) are adjacent and contained in a main plane of extension, to a closed rest configuration, where the circle portions (5) are at least partly superimposed so as to limit the volume in the main plane of extension, and vice versa. Owing to this particular arrangement it will be possible to increase the available space on-board the boat when the rudder is not used and, in so doing, facilitate the movements of the crew.