Seatpost unit, comprising an upper end and a lower end, wherein said upper end is used to facilitate the connection of a bicycle seat and wherein said lower end is configured to be inserted into a seat tube of a bicycle frame, wherein it also comprises: - a hollow tubular body arranged along a longitudinal axis, comprising an inner space and a generally cylindrical outer surface having a longitudinal recess delimited by a longitudinal base; - a guide element comprising an elongated guide body that can be positioned inside the recess in adjustable longitudinal positions and that comprises a first longitudinal end, a second longitudinal end opposite the first longitudinal end, a first face and a second face opposite the first face; - a fixing element for securing the guide element to a seat tube of a bicycle frame; - at least one stop element (20; 40), arranged in a fixed or adjustable position on the tubular body and comprising a stop body (21; 41) at least partially arranged inside the longitudinal recess, the stop body (21; 41) having a first longitudinal end (22; 42) and a second longitudinal end (23; 43) opposite the first longitudinal end (22; 42); wherein - the seatpost unit can adopt a locking position in which said guide body is positioned inside the recess, and in which one of the longitudinal ends (22, 23; 42, 43) of the stop body (21; 41) of a stop element (20; 40) fixed to the tubular body is longitudinally facing and in contact with a longitudinal end (32, 33) of the guide body, characterised in that - the guide body of the guide element comprises a longitudinal elongated through hole, and said fixing element may be placed in different positions along said longitudinal elongated through hole. - a hollow tubular body arranged along a longitudinal axis, comprising an inner space and a generally cylindrical outer surface having a longitudinal recess delimited by a longitudinal base; - a guide element comprising an elongated guide body that can be positioned inside the recess in adjustable longitudinal positions and that comprises a first longitudinal end, a second longitudinal end opposite the first longitudinal end, a first face and a second face opposite the first face; - a fixing element for securing the guide element to a seat tube of a bicycle frame; - at least one stop element (20; 40), arranged in a fixed or adjustable position on the tubular body and comprising a stop body (21; 41) at least partially arranged inside the longitudinal recess, the stop body (21; 41) having a first longitudinal end (22; 42) and a second longitudinal end (23; 43) opposite the first longitudinal end (22; 42); wherein - the seatpost unit can adopt a locking position in which said guide body is positioned inside the recess, and in which one of the longitudinal ends (22, 23; 42, 43) of the stop body (21; 41) of a stop element (20; 40) fixed to the tubular body is longitudinally facing and in contact with a longitudinal end (32, 33) of the guide body, characterised in that - the guide body of the guide element comprises a longitudinal elongated through hole, and said fixing element may be placed in different positions along said longitudinal elongated through hole.