Alysicarpus littoralis Satthaphorn & Leerat. (Fabaceae, Desmodieae), a species confined to Samui Island, Surat Thani province, southern Thailand, is described and illustrated as a species new to science. Initially identified as A. vaginalis (L.) DC., the re-examination of morphological characters revealed distinctions of A. littoralis such as the length of inflorescence internode, length of calyx lobes, the colour of corolla and the type of pods at maturity. The newly proposed species is sometimes similar to A. monilifer (L.) DC. And A. ovalifolius (Schumach.) J. Léonard as it exhibits moniliform pods and lax inflorescence, respectively. The conservation status of the new species is assessed as Critically Endangered (CR, B2(bii+ciii)) following IUCN conservation guidelines since populations are surrounded by human disturbances. A morphological description, line drawing and updated key to species of Alysicarpus in Thailand are presented.