Three corticolous crustose lichens are described from Thailand as new to science. Astrochapsa elongata Poengs. & Lumbsch is characterized by elongated apothecia, a clear hymenium, submuriform ascospores and the lack of secondary metabolites. Graphis khaoyaiensis Poengs. & Lumbsch is characterized by a laterally carbonized exciple, striate labia, a clear hymenium, muriform ascospores and the absence of secondary metabolites, differing from Graphis dichotoma in having smaller ascospores, and not radiately branched ascomata. Phlyctis sirindhorniae Poengs., Vongshew. & Lumbsch is distinguished by apothecia being solitary or sometimes aggregate or fused into groups, muriform ascospores and the presence of norstictic acid, differing from P. agelaea in having larger apothecia and more ascospores per ascus. Descriptions of the new species are provided and discussions on similar species.