An automatic weight stack controller for fitness units adapted to a frame provided with weights piled up is disclosed, wherein the weights are provided with central vertical holes having inner circular grooves and a tube fit into said central holes, said tube being provided with radial holes disposed at the level of each weight wherein balls are housed, so that said balls can be pushed towards the grooves by a piston that slides vertically inside the tube, having conical ends and threaded on a vertical threaded bar centrally to the tube, where there is an eyelet for the steel cable designed to raise the weights, the selection of weights being carried out through a panel of instruments, with a digital display, and buttons to increase or decrease the number of weights.