A safety harness comprises at least one strap, at least one attachment member for connecting the safety harness and a system for closing and adjusting the strap provided with a stationary fastening buckle secured to a first strand of the strap and a movable fastening buckle secured in such a way as to be adjustable along a second strand of the strap. The movable fastening buckle can cooperate with the stationary fastening buckle in an interlocking configuration in which the strap is closed on itself so as to form a closed loop having a perimeter that can be adjusted by adjusting the position occupied by the movable fastening buckle along the second stand. The safety harness comprises a locking part separate from the stationary and movable fastening buckles, secured to the first strand or the second strand of the strap and capable of varying between an active state urging the two, stationary and movable, fastening buckles relative to each other so as to positively immobilise the stationary and movable fastening buckles in the interlocking configuration, and an inactive state in which the two, stationary and movable, fastening buckles can freely adopt and come out of the interlocking configuration.