Phaeocalicium atenitikon is described new to science, growing on twigs of Juglans nigra (black walnut) in central North America. The species is characterized by small ascomata; an exciple of brown, periclinally arranged hyphae; notably small, pigmented, two-celled ascospores with pale septa; and its substrate and distribution. These are the first records of a corticolous Phaeocalicium from intermountain North America south of the Great Lakes region. Differences among similar species are discussed, and a worldwide key is provided for members of the genus with persistently 1-septate ascospores. Keywords: Biogeography, calicioid fungi, corticolous microfungi, Illinois, Kansas, midcontinental United States of America, Missouri, taxonomy.