The victory moves Tunisia to 22 points from their eight qualifiers, 10 points ahead of second-placed Namibia, who have three games left to play.
Tunisia, who are coached by Sami Trabelsi, will make their seventh appearance at the World Cup, which is to be staged in the United States, Mexico and Canada from June 11-July 19, and their third in a row.
It looked as though they would have to wait to seal their spot, but Ben Romdhane was presented with a simple tap-in deep into injury-time after a defensive error from the home side allowed Firas Chaouat to tee him up.
Tunisia are the second African side to seal their place at the global finals after Morocco booked their spot on Saturday.
The Moroccans were 2-0 winners in Zambia on Monday as Youssef En Nesyri and Hamza Igamane scored the goals that keeps up their momentum.
Mozambique kept their qualification hopes alive with a 2-0 win over Botswana in Group G. They are three points behind leaders Algeria, but have played a game more.
Witi and Faisal Banga scored in either half to give them the victory in Maputo.
Nine teams from Africa will qualify automatically for the World Cup, with a 10th place available through an inter-continental play-off.