Tirisporella beccariana, a monotypic marine ascomycete, commonly occurs on the lower base of fronds and rhizomes of the brackish water palm, Nypa fruticans. This fungus has globose to subglobose ascomata, immersed to superficial, ostiolate and periphysate, three layered peridium. It has cylindrical bitunicate-like asci, short peduncle, thick-walled, brown ascospores, 4–7 septate, falcate to lunate, verrucose, apical cell appendaged, hyaline basal cell pointed. Taxonomically it was originally assigned to the Dothideomycetes incertae sedis based on the bitunicate-like asci. During on-going investigations of mangrove fungi in Thailand, the fungus was found in intertidal regions in Trang and Trat provinces, Thailand. Maximum parsimony, likelihood and Bayesian analyses using partial SSU and LSU rDNA sequences place T. beccariana in the Diaporthales, Sordariomycetidae, Sordariomycetes; however, the familial level relationship was unidentified. This genus forms a distinct clade with Thailandiomyces bisetulosus isolated from senescent trunks of the palm Licuala longicalycata in Sirindhorn peat swamp forest, with a high statistical support, although they share few morphological features in common.