As its name suggests this is a robust lichen, though in other respects is very similar to the smaller and much more common Parmotrema perlata. It is grey-green in colour and has a lobed “foliose” habit. The lobes are much larger than those of P. perlata being 5-15mm wide and are more elaborately cut and inrolled, giving a ragged appearance to their margins. Sparse cilia are scattered on the margins (cilia are abundant in P. perlata). Soralia are sparse, rounded and often clustered on small extensions of the cut lobed margins.
It can also be distinguished from P. perlata by chemical tests: the medulla of P.robustum gives a rust-red reaction to Pd and a dirty yellow-brown reaction to K. In P. perlata Pd gives orange and K gives a bright yellow-orange.