We describe a new spider species of the genus Agelenopsis Giebel 1869 (Agelenidae) from adult males and females. Agelenopsis riechertii is found in dry desert scrub habitats in the southwestern United States. While A. riechertii is similar to other sympatric congeners (A. aleenae, A. aperta, and A. spatula) in morphology and courtship behavior, it can be distinguished by an examination of its genitalia. Males possess an embolus that gradually narrows into a twisted tip which displays a convex edge from the lateral view. The female epigyna are harder to distinguish from congeners, but have a flatter anterior atrial edge that lacks strong bi-lobing. The spermatheca in female A. riechertii are also more elongate and comma-shaped than the other species. We describe the sequence of courtship behaviors exhibited by males, including the use of web pulses, abdomen waggling, and web flexing, in which males with fewer pauses, longer waggling bouts, and fewer web flexes are more likely to successfully copulate.