The body is deep, but streamlined. It has a metallic, deep blue back, changing to yellow and silver on the belly. A yellow band extends down the side, and the belly often has about 20 vertical broken lines, a characteristic found on no other tuna, but not always noticeably present on yellowfin tuna. Large fish are easily identified by the long crescent-shaped extensions of the anal and second dorsal fins.

Yellowfin tuna are spawning and eating machines. In spite of being fairly long-lived, some reach sexual maturity during their first year, although most are age 2 or 3 when they first spawn. The spawn several times a year in the open sea at temperatures above 78º F. A 5½-foot long female can lay 8 million eggs. Yellowfin tuna feed on a huge variety of finfish, squid, shrimp, and crabs. They are very efficient sight hunters, but can also actually smell their prey. Fish leave a scent in the water made up of oils, proteins, and amino acids from the slime layer on their bodies. Tiny traces of this wash off of the fish. When yellowfin tuna pick up this scent trail, they actually track down their prey.

Yellowfin tuna make both seasonal and daily migrations. In the Pacific Ocean, they are often found on the edges of island coral reefs during the day. Each night, they travel up to 9 miles offshore to feed and then return to the exact same spot the next day. That's the equivalent, in human terms, of walking 37 miles each night for supper. Tagging studies on tunas in the open sea show similar behavior. A tuna will hang around a floating log or other debris during the day, travel long distances at night, and return to the exact same log the next day.

All species of tuna share some interesting biological characteristics. Fish in general are thought of as "cold-blooded". That means that their body temperature is the same as that of their environment. Tunas (and a few sharks) have developed the ability to control their body temperature through a network of veins and arteries called a "rete mirabile" that traps (and dumps) body heat. Even smaller tunas can maintain temperatures 50º F higher than surrounding water temperatures. This is a huge advantage. For most cold-blooded fish, the colder the water and therefore their body is, the slower and more sluggish they are. Tunas' warmer body temperatures speed up the chemical reactions in their body that produce energy and allows their muscles to contract more quickly. This provides faster swimming speeds and increases their endurance.

No other fish can swim as far or as fast as tunas. Water has a great deal of resistance or drag, so every eight-fold increase in swimming speed takes a 100-fold increase in energy. One structure that reduces drag are the caudal peduncle keels found near the tail fin. These keels reduce water turbulence created by the tail fin and lower the drag by that part of the body. Tunas also have a series of sail-like finlets on the top and bottom of their body behind their fins. These are thought to prevent the development of swirls of water that would spin off the body and tail, allowing the tail fin to work more efficiently in undisturbed water. The first dorsal fin also folds down into a groove on the body to reduce drag when the fish does not need it to maneuver.

Compared to other less active fish, tunas have hearts that are ten times larger for their body weight, pump three times more blood, and have blood pressure three times higher. They also have a much higher proportion of red muscle in their bodies than the average fish, which allows them to cruise at higher speeds more efficiently. Tunas have been observed to swim at 28 mph for long distances. Tunas also have gills that are up to 30 times larger in surface area than those of other fish. Additionally, tuna cannot open and close their gill covers with their opercular muscles to force water over their gills. Flaring gill covers would create drag. This means, however, that tunas must swim or suffocate. They are "obligate ram ventilators." They must swim through the water with their mouths open to stay alive. Oxygen-bearing water is swept over their gills purely due to the movement of the fish. In fact, tunas must swim at a speed of 26 inches per second in order to provide enough water flow to get the oxygen that they need.