A bath tub lifter for seniors or disabled persons comprises a frame to rest on a portion of a bath tub, a seat plate and a backrest in connection therewith, a lifting means for adjusting the height of the seat plate and the backrest between a lowermost and an uppermost end position, the lifting means comprising a driving means mounted to the backrest and a telescopic lifting tube to be actuated by the driving means, a spindle base in connection with the lifting tube for providing support against the frame, and an automatic lock-release device coupling the backrest and the seat plate above the lowermost end position and releasing the backrest and the seat plate in the lowermost end position, the lock-release device comprising at least one lock bolt movably held at the backrest for coupling the backrest and the seat plate above the lowermost end position, and at least one sliding guide formed at the seat plate for actuating the at least one lock bolt.