Pertusaria siamensis is a species of corticolous (bark-dwelling), crustose lichen in the family Pertusariaceae. Found in Thailand, it was formally described as a new species in 2005 by Sureeporn Jariangprasert. The type specimen was collected by the author from the Chae Son National Park (Lampang province) at an altitude of , where it was found in a dry dipterocarp forest growing on Shorea obtusa. The lichen is common in several Thai national parks in various parts of the country, occurring at elevations ranging from . In addition to Shorea, Pertusaria siamensis has also been recorded growing on Dipterocarpus, Ficus, and Vatica. The species epithet combines the old name for Thailand ("Siam") with the Latin suffix -ensis ("place of origin").