An anti-theft closure and immobilization device for a bicycle comprising: - an outer sleeve defining a sleeve axis (M-M), opposite sleeve end openings (13, 14), a sleeve inner surface, a sleeve engagement recess extending from said sleeve inner surface towards outside of the outer sleeve; - a shaft defining a rotation axis (A-A) and a shaft outer surface, said shaft being arranged coaxially to, and inside of said outer sleeve so as to rotate with respect to said outer sleeve about said rotation axis (A-A), said shaft having a first free shaft end and a second free shaft end accessible from said opposite sleeve end openings (13, 14); said shaft having a second free end opening and an inner cavity extending along the rotation axis (A-A) from said second free end opening towards the first free shaft end, said inner cavity defining a cavity side surface; said shaft having a through engagement hole extending between said cavity side surface and said shaft outer surface along an engagement hole axis (F-F); - a locking mechanism arranged inside said inner cavity and actuatable through said second free end opening, comprising a shaft engagement element slidably engaged in said engagement hole between a protruding position with respect to said shaft outer surface and a non-protruding position with respect to said shaft outer surface; - said sleeve engagement recess and said shaft engagement element being mutually positioned so as to align, at a predetermined angular position of the shaft with respect to the outer sleeve, the shaft engagement element with the sleeve engagement recess, so that the shaft engagement element may be partially received in said sleeve engagement recess, in said predetermined angular position of the shaft, thus preventing any further movement of the shaft with respect to the outer sleeve; the anti-theft closure and immobilization device being characterized in that: - the outer sleeve comprising a first sleeve element comprising said sleeve engagement recess, and a second sleeve element, associable each other when approaching each other to form the outer sleeve; and in that - the first sleeve element comprises a first outer fastening portion adapted to be fastened to a bicycle frame, and the second sleeve element comprises a second outer fastening portion adapted to be fastened to the bicycle frame. - an outer sleeve defining a sleeve axis (M-M), opposite sleeve end openings (13, 14), a sleeve inner surface, a sleeve engagement recess extending from said sleeve inner surface towards outside of the outer sleeve; - a shaft defining a rotation axis (A-A) and a shaft outer surface, said shaft being arranged coaxially to, and inside of said outer sleeve so as to rotate with respect to said outer sleeve about said rotation axis (A-A), said shaft having a first free shaft end and a second free shaft end accessible from said opposite sleeve end openings (13, 14); said shaft having a second free end opening and an inner cavity extending along the rotation axis (A-A) from said second free end opening towards the first free shaft end, said inner cavity defining a cavity side surface; said shaft having a through engagement hole extending between said cavity side surface and said shaft outer surface along an engagement hole axis (F-F); - a locking mechanism arranged inside said inner cavity and actuatable through said second free end opening, comprising a shaft engagement element slidably engaged in said engagement hole between a protruding position with respect to said shaft outer surface and a non-protruding position with respect to said shaft outer surface; - said sleeve engagement recess and said shaft engagement element being mutually positioned so as to align, at a predetermined angular position of the shaft with respect to the outer sleeve, the shaft engagement element with the sleeve engagement recess, so that the shaft engagement element may be partially received in said sleeve engagement recess, in said predetermined angular position of the shaft, thus preventing any further movement of the shaft with respect to the outer sleeve; - the outer sleeve comprising a first sleeve element comprising said sleeve engagement recess, and a second sleeve element, associable each other when approaching each other to form the outer sleeve; and in that - the first sleeve element comprises a first outer fastening portion adapted to be fastened to a bicycle frame, and the second sleeve element comprises a second outer fastening portion adapted to be fastened to the bicycle frame.