Gyalecta perithecioidea is a little-known species of saxicolous (rock-dwelling), crustose lichen in the family Gyalectaceae. It is found in Brazil, where it grows on sheltered limestone in the Atlantic Forest.
The lichen was described as a new species in 2020 by Dutch lichenologist André Aptroot. The type specimen was collected by the author from the Serra da Bodoquena (Mato Grosso do Sul at an elevation of . The species epithet refers to its characteristic ascomata.
The thallus of Gyalecta perithecioidea is , with a dull, pale pinkish-green to pale orange colour, and lacks a prothallus. The is green algae. The ascomata are perithecia, located in cavities in the thallus and emerging from the . They are spherical, pale ochraceous to pale orange, and measure 0.3 to 0.4 mm in diameter. The wall is pale and lacks , while the ostiole is orangish and darker than the ascoma wall. The comprises unbranched filled with yellow oil, which is a rare in lichens. are hyaline, numbering eight per ascus, and they are densely irregularly , with only the median septum horizontal. The spores are ellipsoid, measuring 22 to 27 üm by 15 to 16 üm. have not been observed in this species.