Fight! Mighty Jack (戦え！マイティジャックTatakae! Maiti Jakku) is a more comical sequel series to Mighty Jack.
It aired on the same network from July 6 to December 28, 1968, with a total of 26 half-hour episodes, equalling the original in length.
This series has several humorous references to the early Ultra Series.
- The opening scene of the series (with a reverse paint-swirling effect forming "MJ" before a burst of red envelops the scene, with a yellow "Fight! Mighty Jack" superimposed) is almost exactly like that of the original Ultraman.
- One episode of this series is quite notable, as it features a comical guest appearance by Kohji Moritsugu, who played Dan Moroboshi, the alter-ego of Ultra Seven, poking fun at his popular role. He plays a mechanic, who, in one scene, looked as though he was about to transform into Ultra Seven by pulling the Ultra Eye from his pocket to put it on, but the red object he slowly pulls from his pocket is actually a small wrench with which he gets right to work on fixing a machine.