Theretra nessus, the yam hawk moth, is a moth of the family Sphingidae. It was described by Dru Drury in 1773.

It is found in Sri Lanka, India (including the Andaman and Nicobar islands), Nepal, Myanmar, Thailand, Vietnam, southern China, Taiwan, South Korea, Japan, Philippines, Malaysia, Indonesia, northern Australia and New Caledonia. It is a recent immigrant in Oahu.

The wingspan is 90–130 mm. Head, thorax and a stripe down center of abdomen is green suffused with ferrugineous. Thorax is with a lateral grey stripe and abdomen is golden yellow at the sides. Forewings are olive brown and the base with a patch of black and white on the inner margin. A black dot is present at end of cell. A postmedial waved oblique line met by three straight oblique lines from the apex at inner margin. Hindwings are black brown where the anal angle is ochreous, which colour extends towards the apex as a submarginal band. Ventral side is suffused with reddish ochreous.

Larva is bluish green, with a subdorsal line with oblique streaks below it on 4th to 10th somites. Ocellus on the 4th somite is greenish with a black ring. The horn is yellowish. Larvae have been recorded on Amaranthus, Barringtonia, Dioscorea, Amorphophallus, Impatiens, Citrullus, Arathis, Boerhavia, Knoxia, Morinda, Oldenlandia, Pongamia, Spermacoce, Glossostigma and Camellia.