A braking unit (4) for a stairlift, wherein the stairlift comprises a guide rail (2) and a braking carriage (1) slidable on the guide rail (2), the braking carriage (1) comprising the braking unit (4). The braking unit (4) comprises: a safety device (5) which can be moved to a safe condition to engage with the guide rail (2) blocking a sliding of the braking carriage (1); a detection device (6) designed to detect a speed of the braking carriage (1), when the braking carriage (1) slides on the guide rail (2), and is configured to connect to the safety device (5) to cause the movement of the safety device (5) in the safe condition, if the speed of the braking carriage (1) exceeds a predetermined maximum speed. The safety device (5) comprises a safety rotor (501). The detection device (6) comprises: a friction rotor (601), rotating relative to the safety rotor (501) around a first axis of rotation (R1) by the sliding of the braking carriage (1); a variation mechanism (603) acting in conjunction with and coupled to the friction rotor (601) and is configured to connect in a rotationally integral matter the safety rotor (501) with the friction rotor (601) when the speed of the carriage (1) is greater than the predetermined speed. The safety rotor (501) is configured to be rotated around a second axis of rotation (R2), parallel to the first axis of rotation (R1) when the safety rotor (501) and the friction rotor (601) are connected in an integral manner, the safety device (5) also comprising a tapered element (506) fixed to the safety rotor (501) which is configured to be positioned between the friction rotor (601) and the guide rail (2) for locking the safety device (5) in the safe condition and stopping the sliding of the braking carriage (1).