The Atlantic bluefin tuna (Thunnus thynnus), also known as the northern bluefin tuna, giant bluefin tuna (for larger individuals exceeding 150 kilograms or around 300 pounds) and formerly as the tunny, is a species of tuna native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea. Atlantic bluefin have been recorded in the Black Sea in the past, but are now believed to be extinct there. The Atlantic bluefin tuna is a close relative of the other two bluefin tuna species - the Pacific bluefin tuna and the southern bluefin tuna.

Atlantic bluefin tuna are capable of reaching well over a thousand pounds (450 kilograms) in weight, and rival the black marlin and blue marlin as some the largest known bony fish. Throughout recorded history, the Atlantic bluefin tuna has been highly prized as a food fish. Bluefin have been a valuable commercial catch from the time of the Greeks and Phoenicians to the modern era. Besides their value as food and in commerce, the great size that the bluefin can attain and the speed and power they display in their ecological role as apex predators has attracted the admiration and respect of both ancient and modern fishermen, as well as writers, sport anglers and scientists.