The invention relates to a syringe (1) comprising a body (2) delimiting a chamber (C), a piston (3) able to slide in the chamber of the body along a longitudinal axis (X1) of the syringe, and a deformable retention device (4) designed to prevent the piston from leaving the body entirely. The piston has a rod (31), a head (33) and a finger rest (32) which together are all in one piece. The retention device is mounted with force in the chamber and extends around the piston over an angular sector strictly greater than 180° when it is in the assembled position. Moreover, this retention device has external ribs (46) which, protruding, extend parallel to the longitudinal axis (X1), and which are plastically deformable when the retention device is fitted in the chamber. The invention also relates to a method for assembling this syringe.