Abstract
Begonia sibthorpioides subsp. machinchangensis is a newly described taxon restricted to Gunung Machinchang, Langkawi Island, Malaysia. Based on recent collections of this new subspecies and B. sibthorpioides from its type locality of Gunung Jerai, Kedah, detailed morphological comparison has revealed distinguishing characters between these two sandstone-restricted taxa such as indumentum, stipules, bracts, scale-like hairs on tubers, leaf thickness and impressed venation. Descriptions and illustrations for the new subspecies and the type subspecies are provided, and they are also compared to other species, i.e. B. hymenophylla, B. sinuata and B. thaipingensis. Genetic distance based on analysis of chloroplast ndhF-rpl32 sequences supports the delimitation of B. sibthorpioides into two subspecies, and Bayesian phylogenetic analysis showed monophyly of these three species where B. sinuata and B. thaipingensis formed a clade which is sister to B. sibthorpioides. Provisional conservation status assessment using IUCN Red List Criteria indicates B. sibthorpioides subsp. machinchangensis is Critically Endangered (CR) due to its restriction to particular microhabitat of single locality and the population being threatened by new infrastructure development for tourism.