Protomagalhaensia vipera n. sp. (Apicomplexa: Eugregarinida: Blabericolidae) is described from the orange-headed cockroach, Eublaberus posticus (Dictyoptera: Blattaria: Blaberidae: Blaberinae). The oocysts of P. vipera bear extended polar plates or apical corner spines similar to those of Protomagalhaensia granulosae, Protomagalhaensia wolfi, and Protomagalhaensia blaberae. In contrast, the oocysts of Protomagalhaensia richardsoni possess only slight or vestigial apical spines, and apical spines are absent entirely from the oocysts of Protomagalhaensia cerastes. The oocysts of P. vipera are shorter than those of P. wolfi and P. blaberae, but longer than those of P. granulosae (external oocyst length 7.29 µm vs. 7.56 µm, 7.42 µm, 6.87 µm, respectively). The sporozoite bearing cavity of P. vipera oocysts is significantly shorter than any other known species of Protomagalhaensia (internal oocyst length 5.62 µm). All 6 species are also distinguished by unique oocyst length/width ratios. The oocysts of P. vipera are more orbicular than any known species in the genus except P. granulosae. The gametocysts of P. vipera are significantly larger than those of any other known species in the genus (gametocyst length, 405 µm; width, 345 µm). No morphological gamontic feature allows distinction among species of Protomagalhaensia: Species discrimination is morphometric. In particular, satellite total length and satellite deutomerite length are unique and significantly different among species of Protomagalhaensia. The satellites of P. vipera are intermediate in size between P. wolfi and P. richardsoni in both satellite total length (725 µm vs. 523 µm, 798 µm, respectively) and satellite deutomerite length (669 µm vs. 468 µm, 733 µm, respectively).