The new species Scleromitrion sirayanum T.C. Hsu & Z.H. Chen (Rubiaceae) is described and illustrated from Taiwan. S. sirayanum is similar to S. angustifolium (Cham. & Schltdl.) Benth. but easily distinguished by its constantly solitary flowers, salverform corolla, adaxially basally long villous corolla lobes, spreading to ascending persistent calyx lobes, and obconic capsules dehiscent both septicidally and loculicidally into 4 valves. S. sirayanum is so far only found in Nanhua District of Tainan City and evaluated as Vulnerable based on the IUCN Red List criteria. Three new combinations are also proposed for the Hedyotis-Oldenlandia complex in Taiwan.