A decelerating device intended to intercept an end-of-stroke part of a closing path of a mobile member, such as a furniture drawer or door, including: a body fastenable in use to an abutting surface for the mobile member and internally defining a sealed chamber filled in use with a viscous fluid; a pushrod at least in part overhanging from the body and being adapted to cooperate in use with the mobile member, the pushrod being integral with a stem slidingly accommodated in the body against the bias of elastic means between an extracted position of the pushrod and a retracted position; and a piston accommodated in the sealed chamber so as to divide it into two portions hydraulically connected to each other and mechanically connected to the stem; wherein the stem presents a first end integrally connected to the pushrod and a second end opposite to the first, both slidingly accommodated in the body outside the sealed chamber and fluid-tightly towards the sealed chamber itself; the piston is provided with a mobile sealing member to choke the hydraulic communication between the two portions of the sealed chamber only during the stroke of the stem towards the retracted position.