Ascender for ascending on a rope, comprising a U-shaped clamp equipped with two lateral wings delineating an internal space for passage of the rope, and with an arresting surface with spikes against which the rope is blocked in case of a fall. The two wings are provided with two openings arranged facing one another and each having an edge in the form of an inclined ramp performing guiding of a karabiner passing through the two openings. The clamp comprises a mobile control part biased by a spring to an active actuation position of the karabiner along the ramp for the rope to come into contact against the arresting surface with spikes, so as to obtain a first ready-to-block state when ascending on the rope, and a second blocking state of the clamp on the rope in the event of a fall.