Polymeridium longiflavens was formally described by André Aptroot, Cléverton de Oliveira Mendonça, and Marcela Eugenia da Silva Cáceres in 2016.

The type specimen was collected in the Serra de Itabaiana National Park, Sergipe, Brazil, at an elevation of approximately 400 m (1,300 ft).

[2] The thallus of Polymeridium longiflavens is not corticate, dull, continuous, covering areas up to 2 cm (0.8 in) in diameter, and whitish-grey in colour.

It is surrounded by a cottony dark brown hypothallus line approximately 0.4 mm wide and does not induce gall formation on the host bark.

Ascospores are hyaline, have between 9 and 11 septa, fusiform, measuring 57–70 by 12–14 μm, with pointed ends and rounded lumina, and are not surrounded by a gelatinous layer.