Historically,Monotheriumhad been one of the few genera of extinct Phocidae (true seals) that served as a wastebin taxon. Consequently, it did neither aid in understanding phylogenetic relationships of extinct Phocidae, nor in understanding seal diversity in deep time. This urged the reassessment of the genus. Before our review,Monotheriumincluded five different species:Monotherium aberratum,Monotherium affine, andMonotherium delogniifrom Belgium;Monotherium gaudinifrom Italy; andMonotherium?wymanifrom the east coast USA. In this work we redescribe the fossil record of the genus, retaining the type speciesM. delognii.Monotherium aberratumandM. affineare reassigned to the new phocine genusFrisiphoca.Monotherium gaudiniis renamed and considered a stem-monachine (Noriphoca gaudini). The holotype of the monachineM.?wymanirequires further study pending the discovery of new fossil material that could be attributed to the same taxon. Reinvestigating the stratigraphic context reveals thatN. gaudinimost likely represents one of the two oldest named phocid seals, or even the oldest, dated to the late Oligocene–earliest Miocene. Our results allow questioning the widespread idea that Phocidae originated in the western Atlantic and better appreciate their palaeobiogeography during the late Oligocene–Miocene interval in the North Atlantic realm.