Callmander M.W. & S. Buerki (2018). A new species and a new synonym in Pandanus subg. Rykia (Pandanaceae) in Thailand. Candollea 73: 115–120. In English, English abstract. DOI: http://dx.doi.org/10.15553/c2018v731a12The genus Pandanus Parkinson (Pandanaceae) has never been critically revised in Thailand. Most species belong to the taxonomically challenging Pandanus subg. Rykia (Vrise) B.C. Stone. Recent field observations coupled with a review of herbarium material of Thai Pandanaceae (serving as a backbone for the forthcoming family treatment for the Flora of Thailand) have revealed new findings enabling better understanding the taxonomy of Pandanus species in this region. In this article, we formally synonymize Pandanus bifidus H. St. John under Pandanus obconicus H. St. John, two species among the fifteen that Harold St. John described from Thailand in 1963 and 1965. We also describe a new species, Pandanus voradolii Callm. & Buerki, restricted to north-eastern Thailand. The new species resembles Pandanus obconicus in its acaulescent low shrub habit, but differs in its ecology, shape and texture of its leaves, the length of its peduncle and shape and dimensions of its syncarp. Field photographs and a preliminary risk of extinction assessment is provided for the new species.Received: March 3, 2018. Accepted: April 19, 2018. First published online: June 25, 2018.