A bicycle lock for blocking a rotation of the crankshaft (5) of a bicycle is provided at the bottom in a saddle tube (6) of the bicycle close to the crankshaft. The bicycle lock comprises a lock housing (1) accommodating a locking pin (2) and a locking mechanism. The locking mechanism is adapted to fix the locking pin in two positions, wherein it releases or blocks the crankshaft. The bicycle lock is provided with coupling means (12) which are adapted to interact with counter-coupling means being part of the saddle tube in order to have a detachable coupling between the lock housing and the saddle tube. The lock housing (1) is coupled to the saddle tube (6) by a rotating coupling (12). The locking pin in the lower position thereof blocks the rotation of the coupling between the block housing and the saddle tube.