A new species Monoon longipetalum Nurmawati from Sumatra is described and illustrated. It is notable for its long lanceolate and narrowed petals to ca. 9 × 0.8 cm, velutinous on upper surface more densely to the apex of the inner and outer petals; large fusiform to 7 × 2 cm and bluntly apiculated monocarps. This new species is similar to Monoon anomalum (Becc.) B. Xue & R.M.K. Saunders and Monoon borneense (H. Okada) B. Xue & R.M.K Saunders for having fusiform monocarp and long bluntly apiculate, but differ in having fewer clustered flowers in each inflorescence, lanceolate, longer, and narrower petals. Detailed morphological characters together with illustrations and diagnostic comparisons are presented. A key to the 19 species of Monoon in Sumatra is provided.