Gossypiothallon is a fungal genus of uncertain familial placement in the order Arthoniales.

[1] It contains the single species Gossypiothallon appendisporum, a corticolous (bark-dwelling), crustose lichen found in South Solomons.

The type specimen was collected from Kolombangara island at an altitude of 700 m (2,300 ft), where it was found growing on bark.

[2] The lichen has a white, crustose thallus lacking a cortex, and with a cottony (byssoid) texture.

The ascospores, which number eight per ascus, are hyaline, rod-shaped, and have 3 septa; their dimensions are 20–22 by 5.5–6.5 μm.