Adela septentrionella is a moth of the family Adelidae or fairy longhorn moths. It was described by Walsingham in 1880. It is widespread from southern British Columbia and north-western Idaho to the Transverse Range in southern California. The length of the forewings is 4.5–5.7 mm. The forewings are purplish-black with two incomplete thin white bands. The antennae of the males are at least twice as long as the wings. The hindwings are uniformly dark. Adults are on wing from mid April to mid July.