Dieffluvium nidulorum n. sp. is described from smallmouth buffalo, Ictiobus bubalus, collected from Menard Creek in the Big Thicket National Preserve, Texas, U.S.A. The new species is characterized by a scolex with 2 medial acetabula, 4 lateral loculi, and a terminal disc; a uterus that extends far anterior of the cirrus sac in 2 lateral loops; and a very large and muscular external seminal vesicle. Postovarian vitellaria may be present or absent. The new species is a member of the Caryophyllaeidae, and most closely resembles Dieffluvium unipapillatum, Rowardleus pennensis, and some species of Biacetabulum. We amend the genus Dieffluvium to reflect additional information provided by the discovery of the new species, and precisely differentiate its members from other similar taxa.