An anti-theft device for a bicycle that includes wheels disposed on a front fork and on a rear stay by a nut or bolt system and a saddle tube whose clamping axis comprises a nut or a bolt. The device includes a nut or bolt fitted with at least one ratchet notch; a support washer defining a peripheral surface including a raised portion; a saddle collar or clamp; at least one pin that is configured to move inside at least one guide passage defined in the support washer or the saddle collar or clamp; at least one mobile guided pin, configured to configured to move between first and second positions under the weight and rotation of the device, the first position being one in which the bicycle is in a vertical position stood on its two wheels, in which the device permits the tightening/screwing of the nut or the bolt but prevents loosening/unscrewing thereof, the second position being a position one in which the bicycle is stood upside down on its handlebars and saddle, in which the nut or bolt may freely rotate in both directions around its axis, and a seal configured to protect the mobile guided pins against environmental factors.