Macroscopic sarcocysts are often observed in ducks, but at present their taxonomic status remains uncertain because ducks serve as intermediate hosts for several such parasites in the genus Sarcocystis. One such species, Sarcocystis rileyi, was long ago established to involve the northern shoveler duck (Anas clypeata) and the striped skunk (Mephitis mephitis) as its intermediate and definitive hosts, respectively. Here, we employed light microscopy, electron microscopy, and DNA sequencing to more precisely describe diagnostic attributes of parasites presumed to represent S. rileyi occurring in a naturally-infected mallard duck (Anas platyrhynchos). By light and transmission electron microscopy, sarcocysts from the mallard duck resembled the S. rileyi described from A. clypeata. We document 18S, ITS-1, and 28S rDNA sequences from the mallard duck, the first for S. rileyi from any host. Sequences of conserved and variable portions of nuclear ribosomal DNA indicated that S. rileyi is related to, but distinct from, parasites employing opossums as their definitive host (including Sarcocystis neurona and Sarcocystis falcatula). Diagnostic ultrastructural features and nucleotide sequences should aid in future studies and communications regarding this parasitic taxon, which lends itself to experimentation because its sarcocysts are macroscopic and easily excised from infected birds.