Buellia peregrina is described as new to science from small limestone pebbles on exposed ground of the coastal Namib Desert, Namibia. The species is unusual for two reasons: (1) it is the only species in Buellia s.l. currently known to grow euendolithically, i.e., establishing a thallus entirely within its rock substratum, and (2) its apothecia contain a previously unknown blue pigment, here described as peregrina-blue according to its colour and characteristic reactions with HNO3, KOH and HCl. Keywords: apothecial pigments, Buellia peregrina, endolithic lichens, Namibia, peregrina-blue.