Five species of damselflies, Argia sedula, Ischnura ramburii, Enallagma basidens, Enallagma civile, and Enallagma carcunculatum, were collected from Imperial and San Diego counties in California, U.S.A., and were found to collectively host 4 species of actinocephalid gregarines. Setasedecim fursus n. gen., n. sp. (Apicomplexa: Actinocephalidae: Acanthosporinae) is described from the adults of Rambur's forktail Ischnura ramburii (Odonata: Zygoptera: Coenagrionidae) from Sunbeam Lake, Imperial County. This genus is distinguished from the existing genera within Acanthosporinae by oocysts bearing a total of 16 spines: 8 equatorial, 2 at each equatorial vertex, terminal spines inserted at each pole, and 1 at each vertex created by polar truncations. The gregarines recovered are referable to the family Actinocephalidae but are taxonomically distinct from its constituent genera. We describe the new taxon and provide new host and locality records for known actinocephalid gregarines.