The lichen family Thelotremataceae was studied in Thailand and Malaysia (including one specimen from Indonesia). Of the 30 new species recognized, 26 are recorded from Thailand: Myriotrema grandisporum, M. khaoyaianum, M. khuntanense, M. rongklaense, M. subanamaliense, M. subgranulosum, M. subminutum, M. thailandicum, M. whalleyanum, Ocellularia brunneospora, O. diospyrosis, O. flavescens, O. inthanonensis, O. kansriae, O. krathingensis, O. melanophthalma, O. neoleucina, O. peremergens, O. pluriporoides, O. rhicnoporoides, O. subleucina, O. wolseleyana, Thelotrema laemense, T. mongkolsukii, T. phliuense and T. rhododiscum; and 4 from Sarawak: M. coppinsii, M. muluense, O. neoperforata, and O. roseotecta.Additional notes are provided on Ocellularia palaeoamplior Aptroot & Sipman and Thelotrema isidiophorum (Kremp.) Zahlbr.