The bigeye tuna, Thunnus obesus, is an important food fish and prized recreational game fish. It is a true tuna of the genus Thunnus, belonging to the wider mackerel family Scombridae.

In Hawaiian, it is one of two species known as ʻahi; the other is yellowfin tuna.

Bigeye tuna are found in the open waters of all tropical and temperate oceans, but not the Mediterranean Sea.

Bigeye tuna vary up to 250 centimetres (98 in) in length. Its maximum weight probably exceeds 400 pounds (180 kg), with the all-tackle angling record standing at 392 pounds (178 kg). They are large, deep-bodied, streamlined fish with large heads and eyes. The pectoral fins are very long, reaching back as far as the second dorsal fin. They display 13 or 14 dorsal spines.

The bigeye forages in cold and oxygen-poor subsurface waters. Their blood extracts oxygen efficiently even in oxygen-poor conditions. Their vision functions well in low light conditions. The heart has an unusual ability to function effectively while foraging in cold subsurface water. Nonetheless, they must periodically return to warmer surface waters to warm up.