The average adult boomslang is 100–160 cm (3¼–5¼ feet) in total length, but some exceed 183 cm (6 feet). The eyes are exceptionally large, and the head has a characteristic egg-like shape. Coloration is very variable. Males are light green with black or blue scale edges, but adult females may be brown.
Weight varies from 175 to 510 g, with an average weight of 299.4 g.
In this species, the head is distinct from the neck and the canthus rostralis is distinct. The pupils of the very large eyes are round. Boomslangs have excellent eyesight and often move their heads from side to side to get a better view of objects directly in front of them. The maxillary teeth are small anteriorly, seven or eight in number, followed by three very large, grooved fangs situated below each eye. The mandibular teeth are subequal. The body is slightly compressed. The dorsal scales are very narrow, oblique, strongly keeled, with apical pits, arranged in 19 or 21 rows. The tail is long, and the subcaudals are paired. Ventrals are 164–201; the anal plate is divided; and the subcaudals are 91–131.