Balanoglossus gigas is a species of large free-living enteropneust (acorn worm) found in the Atlantic Ocean.

Balanoglossus gigas is recorded as growing to 1.5 metres (4.9 ft) in length, and is thus considered the largest acorn worm currently known.

[6] When handled, B. gigas releases thick, sticky mucus with a strong smell of iodoform, which may serve as a deterrent to predators.

The reaction causing bioluminescence incorporates hydrogen peroxide and a luciferin common to balanglossids.

[13] Balanoglossus gigas are found off the coasts of the U.S. state of Georgia,[14] Brazil,[4] Cuba, and in the Gulf of Mexico.