Mousonnia adpressipilosa D. L. Denham ex Ramírez-Roa (Gesneriaceae), a new solitary-flowered species from southern Mexico and Guatemala, is described and illustrated. It has been collected principally in Chiapas, Mexico, with only one location in the Sierra de los Cuchumatenes, Guatemala. Although the possession of solitary flowers is considered unusual in Moussonia Regel, an increase in the number of species both validly published and undescribed with this condition indicates the opposite. The combination of characteristics that distinguish the new species from other solitary-flowered species are principally the whitish strigose indumentum; the shallowly serrated and revolute leaves; the flat adaxial and abaxial surface of the leaves, adaxial surface green, abaxial surface reddish; and flowers with thick, narrow, slightly ventricose, and gradually widening tubular corollas. A key for the solitary-flowered species of Moussonia is included.