The device (1) is intended to be positioned in a sealed introducer placed in a femoral vein in order to penetrate the left atrium (4) of the heart (3) by passing through the septal wall (5) thereof. The device comprises an assembly for cooperating with a handle under the control of control means for actuation of the assembly, for placing and fixing a reinforcement ring (7) on the mitral valve (2), said assembly being arranged at the end of a manipulation rod (6) and comprising: a bearing member (8) comprising a plurality of arms (8a) connected pivotably to the end of the rod (6) so as to change, under the action of the control means, from a position folded along the rod (6) to a deployed position spaced apart from the rod (6), in order to bear under the mitral valve (2) in a manner uniformly distributed along the periphery of the mitral valve； a counter-bearing member (12) comprising a plurality of arms (12a), at the free end of which arms (12a) the reinforcement ring (7) is arranged, the arms (12a) are connected pivotably to a support (13) disposed coaxially with respect to the rod (6) in such a way as to change, under the action of the control means, from a position folded along the rod (6) to a deployed position spaced apart from the rod (6), in order to realize the counter-bearing on the mitral valve and to position the reinforcement ring (7)； means for removing sutures for fixing the reinforcement ring (7) to the mitral valve.Le dispositif (1) est destiné à être positionné dans un introducteur étanche disposé dans une veine fémorale pour pénétrer dans l'oreillette gauche (4) du cœur (3) en passant par sa paroi septale (5). Le dispositif comprend un ensemble apte à coopérer avec une poignée assujettie à des moyens de commande pour l'actionnement de l'ensemble pour la mise en place et la fixation d'un anneau de renfort (7) sur l'anneau mitral (2), ledit ensemble est agencé à l'extrémité d'une tige (6) de manipulation et comprend: un organe d'appui (8) comprenant
