During an outbreak of Wellfleet Bay virus (WFBV) in common eiders ( Somateria mollissima) from the Cape Cod region of Massachusetts, several birds were diagnosed with trichomonosis consisting of multiple trichomonad species. Six birds were examined, with trichomonads found in ceca in four birds and associated typhlitis in three of these four birds. PCR and DNA sequencing utilizing trichomonad-specific primers targeting the ITS1 region of the ribosomal DNA (rDNA) revealed the presence of Tetratrichomonas gallinarum in the gastrointestinal tracts of five birds and Trichomonas spp. in the livers of two birds, one of which had a dual Te. gallinarum-Trichomonas gallinae infection. Sequence analysis revealed no variation between Te. gallinarum sequences whereas the ITS1 sequences obtained from the other Trichomonas spp. demonstrated the presence of multiple genotypes. One sequence had 100% identity to a Trichomonas sp. previously isolated from a Cooper's hawk ( Accipiter cooperii) and the other sequence was 100% identical to a previously described Tr. gallinae isolate obtained from a Pacific Coast band-tailed pigeon ( Patagioenas fasciata monilis). These findings suggest Te. gallinarum and other Trichomonas spp. possibly contributed to morbidity and mortality in this species. Furthermore, to the authors' knowledge, this is the first report of trichomonad-associated disease in a free-ranging sea duck.