The yellowfin tuna (Thunnus albacares) is a species of tuna found in pelagic waters of tropical and subtropical oceans worldwide.

Yellowfin is often marketed as ahi, from its Hawaiian name ʻahi although the name ʻahi in Hawaiian also refers to the closely related bigeye tuna. The species name, albacares ("white meat") can lead to confusion. The tuna known as albacore in English, is a different species of tuna: Thunnus alalunga. However, yellowfin tuna is officially designated albacore in French, and is referred to as albacora by Portuguese fishermen.

The yellowfin tuna is one of the largest tuna species, reaching weights of over 300 pounds (140 kg), but is significantly smaller than the Atlantic and Pacific bluefin tunas that can reach over 1,000 pounds (450 kg) and slightly smaller than the bigeye tuna and the southern bluefin tuna. Reported sizes in the literature have ranged as high as 239 centimeters (94 in) in length and 200 kilograms (440 lb) in weight. The International Game Fish Association (IGFA) record for this species stands at 388 pounds (176 kg). Angler Kurt Wiesenhutter boated this fish in 1977 near San Benedicto Island in the Pacific waters of Mexico. Two larger fish weighing 395 lb and 399.6 lb were boated in 1992 and 1993 respectively. These remarkable fish stand as the largest rod and reel yellowfin tuna captures thus far. On November 30, 2010, Mike Livingston of Sunland, California^{[disambiguation needed ]} reeled in a 405.2 lb Yellowfin off the tip of the Baja peninsula aboard the vessel the Vagabond. Livingston's 86-inch (2,200 mm) catch, which had a girth of 61 inches (1,500 mm), is still pending verification by the International Game Fish Association to replace Wiesenhutter's 388 pounder as the new all-tackle World Record holder.