Salix prolixa is a species of willow known by the common name MacKenzie's willow. It is native to western North America from Alaska and northwestern Canada to the high mountains of California and Utah. It grows in moist habitat such as riverbanks, springs, and marshes. It is a shrub growing 1 to 5 meters tall. The lance-shaped or pointed, oval leaves are up to 15 cm long, hairless, waxy on the undersides, and accompanied by wide stipules. The inflorescence is a catkin of flowers.