The writing instrument comprises a first barrel, a second barrel, and a self-blocking assembly system for assembling the first barrel with the second barrel. The self-blocking assembly system includes a blocking device for blocking the first barrel with respect to the second barrel in a longitudinal direction and a locking device for locking the blocking device. The locking device includes a slot formed in the first barrel and two ramps carried by the second barrel. The slot extends in the longitudinal direction and has two longitudinal edges. The two ramps are configured to part the two longitudinal edges of the slot progressively in a circumferential direction as the first and second barrels are assembled. A window extends in the longitudinal direction formed between the two ramps, and a lateral button is housed at least partially in the window that includes guide surfaces for guiding the lateral button.