Great Ape is a giant, monkey-like creature that Saiyans can transform into at the full moon to increase their gigantic strength. Dragon Ball GT Perfect Files speculates that Saiyans were originally Great Apes who gained intelligence and took on a humanoid form.

After Goku's Kamehameha manages to overcome Vegeta's Galick Gun, Vegeta becomes furious and decides that he needs to transform to beat him. He soon takes note at the lack of a moon as he had planned the arrival to coincide with the full moon in case transformation was needed. Unfortunately, he reveals that he is able to create a Power Ball to simulate the full moon's light, though this drains his power significantly. He then promptly transforms, overwhelming Goku completely and nearly kills him. His transformation is halted by Yajirobe cutting off his tail. Vegeta's transformation into a Great Ape also causes Goku to finally realize the truth of his own Great Ape transformations in the past were responsible for the death of his Grandpa Gohan and for the damage at the World Martial Arts Tournament.