The lichen is characterised by its cream-coloured to greyish thallus and numerous pseudoisidia, which are small, cylindrical outgrowths on its surface.

Sagenidiopsis isidiata was first scientifically described by lichenologists Göran Thor, John Elix, Robert Lücking, and Harrie Sipman.

The type specimen was collected in the Biotopo del Quetzal in Baja Verapaz, Guatemala, from a montane rainforest habitat at an altitude of 1,600–1,800 m (5,200–5,900 ft).

The presence of diploschistesic acid in S. isidiata is unusual, as this lichen product was previously known to occur only in Diploschistes species.

It is cream-coloured to greyish and loosely appressed to the substrate, and surrounded by a dark brown prothallus, up to 5 mm wide, comprising interwoven and radiating hyphae.