Abstract: Adjustable device to support a chair on a boat seat enabling the chair to be moved sidewise and locked either in a center position or selected offcenter positions on both sides. A wheeled carriage, with bolts connecting it to a swivelable chair, is movable from side to side along a floor plate in an elongated body which is fastened lengthwise by clamps on a boat seat bench or thwart. A manually movable spring-loaded latch pin mounted on the carriage is selectively engageable in any one of a plurality of apertures in the floor plate to lock the carriage in a center position or a selected offcenter position. The clamps are adjustable to accommodate seat benches of different vertical thicknesses and different fore-and-aft depths. A concealed space is provided within the lower part of the body for threaded clamp adjustment members.