Thunnus tonggol is a species of tuna of tropical Indo-West Pacific waters.
It is commonly known as the longtail tuna or northern bluefin tuna. The usage of the latter name, mainly in Australia to distinguish it from the southern bluefin tuna, leads to easy confusion with Atlantic bluefin tuna of the Atlantic and Pacific bluefin tuna of the North Pacific. Compared to these "true" bluefins, Thunnus tonggol is more slender and has shorter pectoral fins.
Thunnus tonggol reaches 145 centimetres (57 in) in length and 35.9 kilograms (79 lb) in weight. Compared to similar-sized tunas, its growth is slower and it lives longer, which may make it vulnerable to overfishing.