Described is a locking stop mechanism for a riflescope that includes at least one rotatable reticle adjustment dial mounted on a scope body. It includes a stop member on the adjustment dial at a preselected position and a lock member on the scope body. The lock member is positioned to engage the stop member upon rotation of the adjustment dial to a preselected setting. The lock member is configured to prevent rotation of the adjustment dial in either direction when engaged with the stop member while allowing free rotation in at least one direction when the lock member is manually displaced and when the stop member is not engaged with the lock member at the preselected setting.