Rhizoplaca ouimetensis is described new to science, growing on outcrops of diabase sills in the Lake Superior region of Ontario, Canada. It is the first known sorediate species of the genus, and a phylogenetic reconstruction based on the ITS and mtSSU markers place it in the R. chrysoleuca group. Morphologically, however, it resembles sorediate, yellow-green species of Lecanora with usnic acid, e.g., L. handelii and L. soralifera, but differs from those in forming larger, often pulvinate or minutely peltate areoles with a well-developed upper cortex and a medulla densely filled with calcium oxalate crystals. Keywords: Lecanorales, lichenized Ascomycota, new species, North America, taxonomy.