Abstract: A system for blocking a plug valve, the system comprising: a plug (1) movable in rotation about an axis (Y-Y); and a blocker part (5); the system being characterized in that: the plug (1) presents an index portion having a first ramp (3) and a second ramp (4) that are arranged around the shaft (2) with oppositely-handed slopes; and the blocker part (5) presents first and second index portions (8, 9) adapted to come selectively into contact with the first and second ramps (3, 4) respectively; the first ramp (3) and the first index portion (8) serving to prevent the plug (1) from turning about the longitudinal axis (Y-Y) in a first direction of rotation, and the second ramp (4) and the second index portion (9) serving to prevent the plug (1) from turning about the longitudinal axis (Y-Y) in a second direction of rotation.