Filsoniana rexfilsonii is a species of saxicolous (rock-dwelling), crustose lichen in the family Teloschistaceae.

The lichen was first formally described in 2007 by the lichenologists Sergey Kondratyuk and Ingvar Kärnefelt; it was initially placed in the genus Caloplaca.

These squamules are generally flat, sometimes slightly overlapping, and are centrally attached, with marginal portions appearing to peel away.

[1] The apothecia (fruiting bodies) of Filsoniana rexfilsonii are 0.3 to 0.7 mm in diameter with a slightly raised margin.

The true exciple is scleroplectenchymatous, measuring 15 to 20 μm thick in the lower lateral and basal portions.

The species is adaptable to both sheltered and sun-exposed environments, ranging from coastal areas to subalpine regions and dry Australian localities.