A movable platform assembly for a boat includes a support structure configured to be connected to the boat, a platform, and a horizontal translation system connecting the platform to the support structure, the horizontal translation system having a pair of main arms rotatable about respective vertical rotation axes, and a pair of main sliders sliding in a transverse direction along a main guide integral with the platform. Each main arm has a proximal end hinged to the support structure and a distal end hinged to a respective main slider. The platform is translatable between a retracted position and an extended position with respect to the support structure.