Phai Liang, a recent bamboo cultivar originating in Thailand and widely introduced to many parts of Malaysia and Singapore, has yet to be diagnosed scientifically. In this study, the partial nuclear Granule Bound Starch Synthase I (GBSSI) gene of Phai Liang was cloned and sequenced to investigate its hybrid origin. Its morphological characteristics were compared with those of its putative parental taxa. Based on both molecular and morphological evidence, Phai Liang is recognized as representing a new nothogenus and nothospecies, ×Thyrsocalamus liang. This circumscribes a hybridization complex with variable morphological characters which reflect different degrees of intergradation between the parental species, Dendrocalamus membranaceus and Thyrsostachys siamensis.