An undescribed species of Phaleria Jack from Sulawesi has been uncovered during ongoing herbarium studies of Thymelaeaceae and is still only known from the type. The species, P. stevensiana Z. S. Rogers, is endemic to a single limestone mountain in South Sulawesi and assigned a preliminary IUCN conservation status of Critically Endangered (CR). Morphologically, it is easily distinguished from the widespread P. octandra (L.) Baill. and P. capitata Jack by its linear-lanceolate leaves (length:width ratios ca. 11–20:1) and distinctive venation pattern, composed of ca. 29 to 35 pairs of secondary veins that diverge from the midrib of the blade at ± 90° angles; the contrasted species have much broader leaves with substantially fewer secondaries. Variation in other characters related to inflorescences, flowers, and fruits is also discussed, as is their taxonomic value. Moreover, all four species of Phaleria occurring in Sulawesi are allopatric, separated geographically and ecologically. The new species, overlooked in several herbaria since 1980, raises the number of Sulawesian Thymelaeaceae species to 10 and the total number of Phaleria species to ca. 25.