Seed coat morphology is examined in 16 taxa of Gentianopsis, Pterygocalyx, and Gentianella, including representatives from 14 taxa of Gentianopsis, to resolve inconsistencies in previous reports and make new observations using a low vacuum mode of SEM. Four characters are proposed to describe variation in seed coat morphology of this group: (1) the outer periclinal walls of testa; (2) sculpting of inner periclinal walls; (3) seed shape; and (4) seed length. The distinctive papillate seeds of many species of Gentianopsis are the result of inflated outer periclinal walls of testa cells. In some species, the outer periclinal walls of the testa are collapsed inwards, revealing previously unreported sculpting on the inner periclinal walls. Seeds are irregularly angular-ovate or fusiform in Gentianopsis and discoid in Pterygocalyx. Seed length varies from 0.22 to 1.0 mm. These data provide another line of evidence for phylogenetic relationships of Gentianopsis and Pterygocalyx to other members of Swertiinae, and illuminate the affinities of fossil seeds from New England that have been assigned to Gentianopsis.