The writing instrument comprises a tubular body (2), a writing tip, a protective element (3) comprising a guide portion (5) axially sliding within the body and extended by a protective end (6), the protective element being movable between a protection position in which the writing tip is protected and a retracted position in order to allow writing. In order to lock the retracted position, the instrument (1) comprises: - a stop (12) formed on the body; - a locking member (7) which engages with the stop to maintain the retracted position; - an unlocking portion (8); - an elastic tab (9) which is connected to the locking member and can be pressed radially inwards in response to pressure applied to the unlocking portion. Displacement of the tab unblocks the locking member, causing the protective element (3) biased by an elastic return element (15) to then slide into the protection position.