A hydraulic coupling device, comprising a coupling unit, a holder block, and a lock nut. The coupling unit is mounted to the holder block by means of the lock nut, which is tightened in a thread at the nose end of the coupling unit. The lock nut has a rotationally symmetrical outer contour. A plurality of grooves distributed around the circumference are recessed in the outer contour. The grooves extend axially inward from the end of the lock nut facing away from the holder block. The invention also relates to a lock nut for a hydraulic coupling device.