Thismia guangdongensis, a fully mycoheterotrophic plant collected from Guangdong Province, China, is proposed and described as a new species. It is assigned to Thismia subsect. Brunonithismia based on having free tepals distinctly dissimilar in shape and size, and by phylogenetic evidence based on molecular data. This new species is morphologically similar to T. breviappendiculata from southern Myanmar but is easily distinguished from the latter by its orange campanulate hypanthium, inner surface of hypanthium with orange transverse bars, and filiform appendage of the inner whorl of tepals. With a plastome 20,379 bp in length and only 14 intact genes, the proposed new species has an extremely reduced plastome at the last stage of degradation, similar to most of the Thismia species previously studied.