Adapter for a windscreen wiper, notably a motor vehicle, having a body of longitudinally elongate shape which is delimited by two, longitudinal and vertical, external lateral faces (29a, 29b) and which is configured to be mounted in a complementary yoke of inverted-U-shaped cross section belonging to a wiper arm, this yoke having a pair of front locking pegs (44a, 44b) and an abutment surface which are able to cooperate with associated parts of the adapter in order to fasten the yoke to the adapter in a removable manner, each of the external lateral faces (29a, 29b) of the body of the adapter having a front locking notch (59a, 59b), each of which is able to receive an associated front locking peg (44a, 44b) of the yoke, and the body of the adapter being provided with a resilient branch bearing an unlocking button which is able to cooperate with the abutment surface of the yoke in order to longitudinally immobilize the adapter in one direction with respect to the yoke, each of the longitudinal lateral faces (29a, 29b) of the body of the adapter having a rear vertical slot (60a, 60b) that is open at its two ends, each slot being able to receive an associated peg (46a, 46b) of the yoke in a sliding manner, each of the longitudinal lateral faces (29a, 29b) having, at its lower end, an associated locking notch (61a, 61b), each rear vertical slot (60a, 60b) opening into the associated notch (61a, 61b), characterized in that the resilient branch extends longitudinally beyond a transverse end face (27a, 27b) of the body of the adapter, the unlocking button being offset vertically downwards with respect to an upper face of the body of the adapter, the unlocking button also being offset longitudinally forwards with respect to the plane of the front transverse end face of the body of the adapter, and in that the resilient branch, in the mounted and fixed position of the yoke on the adapter, extends next to an internal face of a front transverse end wall of the yoke. each of the external lateral faces (29a, 29b) of the body of the adapter having a front locking notch (59a, 59b), each of which is able to receive an associated front locking peg (44a, 44b) of the yoke, and the body of the adapter being provided with a resilient branch bearing an unlocking button which is able to cooperate with the abutment surface of the yoke in order to longitudinally immobilize the adapter in one direction with respect to the yoke, each of the longitudinal lateral faces (29a, 29b) of the body of the adapter having a rear vertical slot (60a, 60b) that is open at its two ends, each slot being able to receive an associated peg (46a, 46b) of the yoke in a sliding manner, each of the longitudinal lateral faces (29a, 29b) having, at its lower end, an associated locking notch (61a, 61b), each rear vertical slot (60a, 60b) opening into the associated notch (61a, 61b), characterized in that the resilient branch extends longitudinally beyond a transverse end face (27a, 27b) of the body of the adapter, the unlocking button being offset vertically downwards with respect to an upper face of the body of the adapter, the unlocking button also being offset longitudinally forwards with respect to the plane of the front transverse end face of the body of the adapter, and in that the resilient branch, in the mounted and fixed position of the yoke on the adapter, extends next to an internal face of a front transverse end wall of the yoke.