A motor vehicle exhaust line silencer includes: an upper shell (2) having a substantially flat upper end wall (16); and a lower shell (4) having a substantially flat lower end wall (24) parallel to the upper end wall (16), the upper and lower shells (2, 4) delimiting between them an internal volume (6) of the silencer (1). The silencer (1) includes at least an internal partition wall (12) substantially parallel to the upper (16) and lower (24) end walls and arranged in the internal volume (6), the upper (16) and lower (24) end walls having respective reliefs (48, 50, 52) projecting into the internal volume (6) and cooperating to immobilize the internal partition wall (12).