Peltigeralean lichenized fungi were surveyed in the mangroves of Trat Province at the eastern coast of Thailand. Eleven species were found belonging to the genera Coccocarpia, Lepidocollema, Leptogium and Physma. Among them, the genus Coccocarpia was most abundant and diverse. Lepidocollema wainioi was also recorded here from Thailand for the first time. Based on the current classification of the peltigeralean fungi the phylogenetic placement of the suborder Collematineae was evaluated using a combined dataset of nuclear ITS and mitochondrial SSU rDNA sequences. Phylogenetic analyses using maximum likelihood and Bayesian approaches revealed issues with the species delimitation in the genera Coccocarpia and Lepidocollema, where the data indicated non-monophyletic lineages. Additional studies with extended sampling will be necessary to address species boundaries in these lichens.