Parmotrema cactacearum is a rare species of corticolous (bark-dwelling), foliose lichen in the family Parmeliaceae.

The type specimen was collected from Pinta Island at an altitude of 289 m (948 ft), where it was found growing on an old cactus pad in an open woodland.

[2] The upper thallus surface of Parmotrema cactacearum is dull, immaculate, and pale greenish yellow, without cracks, and can be sparsely to abundantly isidiate.

The cylindrical isidia are mostly simple to sparsely branched, 0.05–0.1 mm in diameter, and concolorous with the thallus, except for their dark brown tips.

The lobes are small to moderate-sized, angular, and more or less flattened, with sparsely to abundantly ciliate margins, with short and stout black cilia that are mostly simple, rarely branched and typically 0.2–0.5 mm long.