A reversible locking device 1 for a tensioner 100 of a timing chain 20 includes a locking device body 3, integral with the tensioner 100 and protruding with respect to the body of the tensioner 100, which has a shaped slot 2. A perforated protrusion 4 is integral with the shoe 5 carried by the piston 6 and protrudes with respect to the shoe 5 so that its hole is aligned with the shaped slot. A rod 7 has a first part 8 with a shape complementary to that of the shaped slot 2 and a second part 9 bearing at one end a conical portion 10 adapted to couple it irreversibly to the perforated protrusion 4 integral with the shoe 5.