We report a specimen of Cardicola forsteri Cribb, Daintith, and Munday, 2000 (Digenea: Sanguinicolidae) from the lumen of the heart of a northern bluefin tuna, Thunnus thynnus (Linnaeus, 1758) (Scombridae), that was 218 cm in total length (TL) and caught in the northwest Atlantic Ocean 12 km south of Cape Lookout, North Carolina. The hearts of 12 similarly sized northern bluefin tuna (127–262 cm TL) from George's Bank, northwest Atlantic Ocean, were not infected. This is the first report of C. forsteri from a wild host and of a sanguinicolid from any scombrid in the northwest Atlantic Ocean. Bluefin tuna is cultured in sea cages because of its highly prized flesh, and the fact that a blood fluke infects this host is significant because some blood flukes have been identified as serious pathogens of cage-cultured fish.