Thelidium pacificum H. Harada is described as new from Japan. It is unique by growing on rocks in the intertidal to splash zones, and is morphologically characterized by having ±brownish, continuous thin thallus, small perithecia (ca. 0.1 mm diam.), short and thin periphyses (7-10 µm long, 1-1.5 pm diam.), and transversely 1-septate narrow ascospores (12-15 x 3-4 µm)