Monoon vietnamensis N.S. Lý, a new species of Annonaceae, is described and illustrated from Mount Dầu, Quảng Ngãi Province, central Vietnam. It is morphologically similar to M. bornensis and M. anomalum, but differs by much larger and oblongelliptic to obovate-elliptic leaves, with 12–14(16) pairs of nerves and a cuneate leaf base; much longer and narrowly ovate to linear ovate petals, with sparsely scattered short hairs outside; inner petals slightly longer than the outer petals; and much longer fruiting pedicels. A key to the species of Monoon in Vietnam is provided.