A device (10) for treating flat feet, comprising an expandable block (11) with an axial seat (12) for an expander insert (13) adapted to be inserted in the axial seat (12), the expandable block (11) comprising two opposite end parts (14, 15) and a central part (16), the two opposite end parts (14, 15) being configured to be deformed so as to increase their own radial space occupation with respect to the central part (16) when the expander insert (13) is arranged in the axial seat (12).
