The crested treeswift is a large slender bird at 23 cm (9 in) length. This species is dove grey above and white below. The long swept-back wings are a darker grey above. This treeswift has a crest and a long, deeply forked tail. The adult male has orange sides to its face. Young birds have a dark grey head and wings, but the rest of the soft plumage is much streakier than that of the adults. The call of this species is a harsh kee-kyew.