The former Watford forward scored in the third minute as the Super Eagles won at the Al Salam Stadium on Wednesday.
It was his fifth goal of the tournament and he is the competition's top scorer ahead of Friday's final between Algeria and Senegal.
Nigeria have finished third eight times and have won the trophy three times, the last coming in 2013.
Ighalo cashed in on a defensive error when he stabbed in from two yards after goalkeeper Moez Ben Cherifia spilled Jamilu Collins' cross and it hit defender Yassine Meriah.
It proved to be the winner as Ferjani Sassi and Ghailene Chaalali missed decent chances for Tunisia.
Cherifia also needed to thwart Samuel Chukwueze in the 63rd minute and he stopped Samuel Kalu making it 2-0 when he saved his late free-kick.