Roccellinastrum lagarostrobi is a rare species of byssoid (wispy) lichen in the family Byssolomataceae.

Found in Australia, it was formally described as a new species in 1990 by lichenologist Gintaras Kantvilas.

The type specimen was collected from Pine Creek, north of Greystone bluff (Tasmania) at an altitude of 140 m (460 ft); here, along the bank of a stream, at the edge of a rainforest, the lichen was found growing as an epiphyte on leafy twigs of the endemic conifer Lagarostrobos franklinii.

It has a white, cottony (byssoid) thallus that forms irregularly shaped tufts typically measuring 2–4 mm wide.

It is only known to occur at a few locations in Tasmania; its diminutive size and somewhat inconspicuous appearance makes it easy to miss.