We used microsatellite genotyping to determine the genetic relatedness of 7 apparent twin dyads of Weddell seals (Leptonychotes weddellii) born in McMurdo Sound, Antarctica. Genetic evidence for twinning in wild pinnipeds has not been reported previously. A review of 14 years of demographic data combined with pathological exams, behavioral observations, and molecular genetic evidence suggests that twinning is extremely rare in Weddell seals and that females do not wean both pups in the wild. The incidence of live twin births was about 0.1% (2/1,439 births), recorded over 3 seasons in Erebus Bay, McMurdo Sound. Additionally, a single case of true twinning was documented from 23 known pregnancies observed in an isolated population of Weddell seals located within McMurdo Sound. The 3 twin sets were dizygotic full siblings, 1 nontwin dyad represented a case of adoption, and the 3 remaining putative twin sets were identified as instances of foster nursing. These results indicated that observation of mother–offspring behavior was not a reliable method for identifying a twin birth in this species. Use of genetic techniques to verify presence of twins in species with low or unknown twinning rates offers the opportunity for a refinement of estimates in studies of reproductive success, fostering behavior, and adoption.