The yellowfin tuna is one of the largest tuna species, reaching weights of over 300 pounds (136 kg), but is significantly smaller than the Atlantic and Pacific bluefin tunas that can reach over 1,000 pounds (454 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 (94.1 in) in length and 200 kilograms (441 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.

The second dorsal fin and the anal fin, as well as the finlets between those fins and the tail, are bright yellow, giving this fish its common name. The second dorsal and anal fins can be very long in mature specimens, reaching almost as far back as the tail and giving the appearance of sickles or scimitars. The pectoral fins are also longer than the related bluefin tuna, but not as long as those of the albacore. The main body is very dark metallic blue, changing to silver on the belly, which has about 20 vertical lines.

Yellowfin tuna prey include other fish, pelagic crustaceans, and squid. Like all tunas their body shape is evolved for speed, enabling them to pursue and capture fast-moving baitfish such as flying fish, saury and mackerel. Schooling species such as myctophids or lanternfish and similar pelagic driftfish, anchovies and sardines are frequently taken. Large yellowfin prey on smaller members of the tuna family such as frigate mackerel and skip jack tuna.

In turn, yellowfin are preyed upon when young by other pelagic hunters, including larger tuna, seabirds and predatory fishes such as wahoo, shark and billfish. As they increase in size and speed, yellowfin become able to escape most of their predators. Adults are threatened only by the largest and fastest hunters, such as toothed whales, particularly the false killer whale, pelagic sharks such as the mako and great white, and large blue marlin and black marlin. The main source of mortality, however, is industrial tuna fisheries.