Anti-theft bicycle locking device, wherein the bicycle has a saddle column (4), a saddle (3) and a rear frame tube (2), the device has a foldable locking loop that can be wrapped around the bicycle and a stationary object, the loop comprises a plurality of 5 loop portions of which each has a predetermined length, a first loop portion is constituted by the saddle column (4) being a hollow tube, and a second loop portion (8, 16, 20) that has an outer diameter smaller than the diameter of the saddle column (4) and has a first end slidably connected to and moving in the interior of the saddle column (4), a closure member (11, 17) in the remote end part of the saddle column (4)10 preventing the removal of the second loop portion (16, 20) from the saddle column (4), a lock (7) is attached to an upper portion of the saddle column (4) close to the saddle (3), and a lock part (9) is attached to a free end of the last portion of the loop, the lock part (9) can be fitted in and engaged by the lock (7), and said loop portions can be pushed to take a compressed state and pulled to form the loop, an in compressed15 state said loop portions except for the saddle column (4) are positioned in the hollow inner space of the saddle column (4) and the rear tube (2), and the loop comprises at least one further loop portion attached to the remote end of the second loop portion (8, 16, 20), and the loop has a minimum length (LL) in fully unfolded state which is at least as long as the combined length of the rear frame tube (LFT) and twice the length20 of the saddle column (LSC), in equation: LL>LFT+2*LSC.