Phaeographis xanthonica is a species of corticolous (bark-dwelling), crustose lichen in the family Graphidaceae.

The type specimen was collected in Itatiaia (Serra da Mantiqueira, Rio de Janeiro) at an altitude of 1,750 m (5,740 ft).

[1] The thallus of Phaeographis xanthonica is smooth, matt, whitish-grey to whitish-beige, and lacks a prothallus.

Its ascomata are lirelline in form, curved with tapered ends, and measure 0.5–4 mm long.

The ascospores have a transverse septum that divides it into four cells; their dimensions fall into the range 11–21 by 5–7 μm.