Gatchaman II(科学忍者隊ガッチャマンII Kagaku Ninjatai Gatchaman Tsū, Science Ninja Team Gatchaman II is the direct sequel to Science Ninja Team Gatchaman, set two years after the first television series. After the defeat of Leader X, he exacts his revenge by mutating a young girl into Gel Sadra, making her the new leader of Galactor. While Galactor returns, Dr. Nambu and the International Science Organization bring the Science Ninja Team Gatchaman back into active duty.
This series and Gatchaman Fighter would be the basis of an English adaptation titled Eagle Riders, which was produced by Saban Entertainment and aired in 1996. Only thirteen episodes were broadcast in the USA, though the entirety managed to air overseas.
A Korean compilation movie titled Eagle 5 Brothers (독수리 5 형제, Dokksuri Hyeongje) was released in 1980, and is infamous for rotoscoping and plagiarizing the Gatchaman II animation, splicing together various bits of the series to create a 70 minute plot. This movie received a Spanish dub in turn, titled "Space Heroes" ( "Heroes del Espacio").
- Early plans for a Gatchaman sequel, dated around 1976, had X save Joe from his certain death at Cross Karokoram and refurbish him with cybernetic parts, along with brainwashing him to be the villainous "Space Joker" who would antagonize the Science Ninja Team. Meanwhile, Joe's long-lost younger brother named Jack would join the Science Ninja Team as G-6/The Hawk, and would ultimately wind up having to fight against the Space Joker. This treatment was called "Gatchaman 2" (note the lack of Roman numerals).
- Another treatment was titled "New Science Ninja Team Gatchaman" (新科学忍者隊ガッチャマン Shin Kagaku Ninjatai Gatchaman), or "New Gatchaman" for short. In this concept, X would rescue a 25-year old warrior woman named Sadomadonna and recruit her to head the revived Galactor organization. A shady figure named Dr. Einstein would have created an android duplicate of Joe to rejoin the Science Ninja Team, with the android carrying the memories of the original deceased Joe Asakura and believing himself to be the man until learning of his true nature and sacrificing himself to destroy X.