The folding clasp for an interchangeable watch strap comprises at least two arms articulated together and foldable against each other, provided with a fixing device to hold the clasp in closed position. The free end of one (1) of the arms (1, 3) is provided with a securement device for the end of a strap tongue (11) comprising a guide (8) having in cross-section a general U shape, an axle (9) connecting the two lateral wings of the guide (8) and holding members (10) adapted to coact with the upper surface of the tongue (11) of the strap. The free end of the other arm (3) of the clasp is pivoted on a cover (14) with the help of a screw (12), this cover (14) having lateral portions (13) interconnected by an axle (15). The cover (14) of the clasp comprises retaining members (16) adapted to coact with the upper surface of a second tongue (17) of the strap.