Yellowfin tuna are epipelagic fish that inhabit the mixed surface layer of the ocean above the thermocline. They are normally a schooling fish and stay in that immediate school. Sonic tracking has found that although yellowfin tuna, unlike the related bigeye tuna, mostly range in the top 100 m (330 feet) of the water column and penetrate the temperature barrier of the thermocline relatively infrequently, they are capable of diving to considerable depths. An individual tagged in the Indian Ocean with an archival tag spent 85% of its time in depths shallower than 75 m but was recorded as having made three dives to 578 m, 982 m and an incredible 1160 m. Deeper diving and cruising behaviour seems to happen more often in the daytime, changing to shallower swimming behaviour at night, probably in response to the vertical movement of prey items in the deep scattering layer.

The yellowfin tuna is one of the largest tuna species, reaching weights of over 300 lb, but still significantly smaller than the Atlantic and Pacific bluefin tunas that can reach over 1,000 lb and slightly smaller than the bigeye tuna and the southern bluefin tuna. Reported sizes in the literature have ranged as high as 239 cm (94 inches) in length and 200 kg (440 lb) in weight. The International Game Fish Association (IGFA) record for this species stands at 388 lb 12 oz. This fish, caught by angler Kurt Wiesenhutter in 1977 near San Benedicto Island in the Pacific waters of Mexico, is the largest yellowfin tuna captured in accordance with the strict IGFA rules. 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 of the yellowfin tuna 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 often travel in schools composed of fish of similar size. They will sometimes school with other species of tuna and mixed schools of small yellowfin and skipjack tuna, in particular, are commonplace. They are often associated with various species of dolphins or porpoises, as well as with larger marine creatures such as whales and whale sharks. They will also associate with drifting flotsam such as logs and pallets, and sonic-tagged individuals have even been tracked following moving vessels. Tracked Hawaiian yellowfin have also been observed to associate with anchored FADs (Fish Aggregation Devices) and with certain sections of the 50-fathom curve.

Although mainly found in deep offshore waters, yellowfin tuna can be found close to shore when suitable conditions exist. Mid-ocean islands such as the Hawaiian archipelago, other island groups in the Western Pacific, Caribbean and Indian Ocean, as well as the volcanic islands of the Atlantic such as Ascension Island often find yellowfin feeding on the baitfish these areas concentrate very close to the shoreline. Yellowfin tuna will also venture well inshore of the continental shelf when water temperature and clarity are suitable and when there is an abundant food source to exploit.