Bacidia thiersiana is described as new to science from scattered locations throughout southeastern North America, including the southern Appalachian Mountains and Coastal Plain. The species is characterized by its frequent occurrence on and near thalli of Bathelium carolinianum, minutely areolate, esorediate thallus with small, pale to tan, biatorine apothecia and the production of lobaric acid. In addition to comparison with other morphologically similar species that produce lobaric acid and other fluorescent secondary compounds, Bacidina crystallifera is discussed and illustrated in detail.