Oocysts of a coccidium found in the small intestine of a road-killed opossum, Didelphis marsupialis (Didelphimorphia: Didelphidae), are described and identified as Eimeria marmosopos. Sporulated oocysts (n  =  57) were subspherical to spherical and measured 21.9 × 22.1 (20–25 × 20–25); oocyst wall rough and striated, yellowish and bilayered; micropyle, polar body, and oocyst residuum all absent, but some oocysts have a polar granule. Sporocysts were ovoidal, 9.8 × 8.7 (8–13 × 7–12), with both a conspicuous Stieda body and sub-Stieda body, but para-Stieda body was absent. The sporocyst residuum was composed of 2–5 globules (1–4 wide) and several smaller ones. Sporozoites (n  =  10) were comma-shaped, ~7 × 2.7 and had 1 refractile body at their wider end and a centrally located nucleus. These morphological characters were compared with other Eimeria species found in related genera of New World marsupials; based on oocyst morphology, we identified this species as Eimeria marmosopos Heckscher, Wickesberg, Duszynski, and Gardner 1999. This finding constitutes both a new host genus and new geographical record for this species.