This species is distinguished by its shiny, olive-green thallus and hemispherical ascomata with pointed ascospores that are consistently hyaline, fusiform, and have three internal partitions (septa).

[2] The thallus of Porina microtriseptata is corticate, smooth, thin, shiny, and continuous, covering areas up to 2 cm in diameter.

The colour of the thallus is olive green, surrounded and partly dissected by a thin black prothallus.

They are arranged in two bundles in the ascus, and surrounded by a gelatinous sheath approximately 1.5 μm wide.

[2] This species has been found on a tree in a wet lowland tropical rainforest and at the time of its original publication was known only to occur in Sri Lanka.