A species of Serendip Brooks and Barriga, 1995, Serendip danbrooksi n. sp., is described from Mexico as a parasite of Rhinoptera steindachneri Evermann and Jenkins, 1891. The new species differs from Serendip deborahae, the type and only other known member of the genus, by having bothridia subdivided by 2 septa, 1 simple and 1 bifurcating, rather than 3 septa, 2 simple and 1 bifurcating, and by having 37–61 testes versus 64–116 testes, respectively. In general, S. danbrooksi n. sp. is smaller than S. deborahae in the number of proglottids (average 77 vs. 150, respectively) and length (maximum length 15.3 mm vs. 60.0 mm, respectively). Clarification of the details of some previously described structures is discussed.