The lichen genus Thalloidima (Ramalinaceae, Ascomycota) is reported as new to China, with specimens primarily collected from the Qinghai-Tibetan Plateau. Through an integrative approach combining morphological, chemical, and molecular phylogenetic analyses of nrDNA ITS sequences, three species new to science are described: T. lobulatum Yu X. Gan & Xin Y. Wang, T. qinghaiense Yu X. Gan & Xin Y. Wang, and T. triseptatum Yu X. Gan & Xin Y. Wang. Additionally, T. sedifolium and T. squamatum are recorded in China for the first time. These discoveries bring the total number of recognised species within Thalloidima to 21. Critical diagnostic characters, including ascospore septation, thallus pruina development, and substrate specificity, are detailed and illustrated. A comprehensive taxonomic key to all currently known species of the genus is also provided. Key words: Lichenized fungi, Taxonomy, Qinghai-Tibetan Plateau, New genus record, Phylogeny.