Taxa in the Gyromitra gigas species complex were previously studied and their taxonomy resolved. During ongoing studies in this group, cryptic speciation was discovered in G. gigas. Sequences of the ITS and LSU regions from 75 specimens were included in maximum likelihood and Bayesian phylogenetic analyses to establish species boundaries and resolve species relationships. Sequence similarity comparisons were also conducted between the two ribosomal markers and between the ITS1 and ITS2 regions. Gyromitra gigas sensu stricto and two additional species were discovered within the G. gigas clade. Gyromitra ussuriensis was rediscovered as a distinct taxon and removed from synonymy under G. gigas. It occurs in central and eastern Asia, whereas G. gigas occurs mostly in Europe but also extends into central Asia. A neotype is designated for G. ussuriensis. A new species, Gyromitra americanigigas, is described and illustrated from eastern North America. Although morphology and the LSU exhibited little variation among the three species, the ITS1 and ITS2 regions displayed similar interspecific sequence variability around 0.5-1%, which is sufficient for species identification at the molecular level.