The lichen genus Arthothelium A. Massal. (Arthoniaceae) in Thailand is presented. The investigated specimens were collected from Nakhon Si Thammarat province and obtained from RAMK herbarium. Two species are reported together with taxonomic description, distribution and ecology of each species, and an identification key is provided. One species, Arthothelium nilamburense Makhija & Patw., is a new record for Thailand.