A steering control system for a watercraft includes a device locking a steering tiller, which can be activated or deactivated for maintaining the tiller in, or moving it from, a predetermined angular position. The locking device includes a hydraulic cylinder having a rod fastened to the transom of the watercraft and a cylindrical body movable along the rod and connected to a steering arm of the motor or of the rudder member. A closed circuit, opened or closed by a valve, flows oil between the two chambers of the hydraulic cylinder and the locking device can be activated or deactivated by an actuator that opens or closes the valve. An interposition element between the steering tiller and a watercraft direction changing member is pivotably articulated to the tiller, and a member transforms the pivoting movement of the tiller relative to the interposition element into commands actuating the valve.