The trinucleid trilobite Bancroftolithus Baldis and Pöthe, 1995 includes a group of species from Sandbian strata of the Argentine Precordillera. The monotypic Hunickenolithus Baldis and Pöthe, 1995 from the Las Aguaditas Formation at its type locality in San Juan Province is placed in the synonymy of Bancroftolithus. Of four nominal taxa from the Las Aguaditas Formation, B. pozensis Baldis and Pöthe, 1995, and B. hughesi Baldis and Pöthe, 1995 are retained, and both are revised based on new collections from the type locality that include ontogenetic series from the protaspid period. Two protaspid stages are recognized in both species. Three additional species (Bancroftolithus alki n. sp., B. cf. B. pozoensis, Bancroftolithus sp.) from broadly coeval strata in the Las Plantas Member of the Las Vacas Formation in La Rioja Province are identified as the closest relatives of the Las Aguaditas species. Bancroftolithus alki also occurs in the Los Azules Formation in San Juan Province. The five species included in this regional clade exhibit significant morphological differences in the lower lamellae, an unusual feature among the Trinucleidae. Bancroftolithus is evidently most closely related to the Australian Katian genus Parkesolithus. The two are not convincingly assigned to any of the existing trinucleid subfamilies.