Ceramothyrium ryukyuense (Chaetothyriaceae) is described as a new species from Okinawa, southern Japan. It is characterized by subglobose minute ascomata (up to 110 μm diam.) covered with a brown mycelial pellicle, small ellipsoid 1(–2)-septate ascospores (8.7–13.8 × 3.4–4.9 μm) within a small ascus (20–30 × 13–17 μm), and a lichenized thallus. This species resembles non-lichenized Brazilian species, C. paiveae and C. philodendri, in producing minute ascomata and 1–4 or 7 septate ascospores. However, besides its lichenized status, C. ryukyuense is distinguished by its smaller asci (up to 30 μm long in C. ryukyuense vs. 30–42 μm long in C. paiveae, and 50–100 μm long in C. philodendri), and predominantly 1-septate ascospores in C. ryukyuense, whereas multi-septate in C. paiveae and C. philodendri. It was collected on a living leaf of Arecaceae in the subtropical forest near the seashore. In a phylogenetic tree based on nuITS and nuLSU sequences, C. ryukyuense formed a sister clade to Ceramothyrium exiguum which is known as an anamorphic species. DNA sequences of C. paiveae and C. philodendri, morphologically similar species to C. ryukyuense, were not available in this study. Algal cells distant from the perithecium exhibited continuous branching, while those near the perithecium were strongly deformed into a spherical shape and were partially unicellular. The photobiont of C. ryukyuense is suggested to be a species of Trentepohliales, inferred from a phylogenetic analysis based on the rbcl sequence. Ceramothyrium ryukyuense is the first report of a lichenized lineage within Chaetothyriaceae. Key words: Arecaceae; Asia; Chaetothyriales; foliicolous lichen; lichenization; subtropics; symbiosis; Trentepohliales.