Marchantia longii R.L.Zhu, Y.L.Xiang et L.Shu sp. nov. (Marchantiaceae) from northwestern Yunnan, China, is described as new to science. It is similar to Marchantia emarginata, but can be distinguished by the large epidermal pore ((108−)150−210 μm in diameter), the acute to obtuse inner margin of surrounding cells of pores in vertical section, the female receptacle dissected (0.2−0.4 of diameter) into 5−7 subquadrate to oblong lobes truncate or rounded apically, and the entire to weakly dentate margin of the appendage of the median scale. A phylogenetic analysis based on DNA sequences of rbcL, trnL–F and nrITS1–5.8S confirms M. longii as a sister to the clade of M. emarginata subsp. tosana M. inflexa M. papillata subsp. grossibarba, and this new species is placed in Marchantia subg. Clamidium.