Jemaa, an injury doubt before the tournament, won it on the stroke of full-time after cutting inside two defenders to power the ball home from close range.
It gave Tunisia a second Group C win after their 2-1 success over Morocco.
Tunisia's left winger Youssef Msakni dribbled past four defenders before placing the ball past goalkeeper Daouda Kassaly after four minutes.
"It was important for us to our first goal of the tournament," said the journeyman striker who plays in the Swedish third division.
Both sides hit the woodwork before Jemaa's goal. Tunisia might have made it 3-1 deep into stoppage-time when Oussama Darragi hit the underside of the bar but the ball bounced back into play.
"It was a hard game, they really made us struggle in the first half. But we got better in the second half," said Tunisia captain Karim Haggui.
Gabon dramatically beat Morocco 3-2 later on Friday in Libreville, ensuring Tunisia's progression to the quarter-finals.
