Phaeographis xanthonica is a species of corticolous (bark-dwelling), crustose lichen in the family Graphidaceae. Found in Brazil, it was formally described as a new species in 2009 by Klaus Kalb and M. Matthes-Leicht. The species epithet refers to the presence of the xanthone substance lichexanthone. The type specimen was collected in Itatiaia (Serra da Mantiqueira, Rio de Janeiro) at an altitude of . It has also been recorded from Costa Rica.
The thallus of Phaeographis xanthonica is smooth, , whitish-grey to whitish-beige, and lacks a prothallus. Its ascomata are in form, curved with tapered ends, and measure 0.5âÂÂ4 mm long. The are black and bordered by a somewhat thick and split . The have a transverse septum that divides it into four cells; their dimensions fall into the range 11âÂÂ21 by 5âÂÂ7 üm.