Monday's victory in Malabo moves Tunisia to 22 points from their eight games in qualifying Group H, 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.
Tunisia are the second African side to seal their place at the global finals after Morocco booked their spot on Saturday.
In another Group H match, Malawi came back from two goals down to draw with Liberia 2-2 with goals from Gabadinho Mhango and Chawanangwa Kaonga in the 72nd and 80th minutes.
In Group G, Algeria played to a goalless draw at Guinea and remained top of the group with 19 points, four more than Uganda. Uganda kept up the pressure by beating Somalia 2-0 to be level on 16 points with Mozambique. Mozambique beat Botswana 2-0.
Madagascar clinched a convincing 3-1 win against Chad to be on 16 points, second to Ghana in Group I. Ghana boosted their hopes of qualifying as the group winner by topping Mali 1-0 in Accra.
Morocco made it seven wins out of seven matches after a 2-0 result at Zambia. Guinea-Bissau beat Djibouti 2-0.
Libya defeated Eswatini 2-0 in Group D to be on 14 points, two behind leader Cape Verde, which has a game in hand.
Nine teams from Africa will qualify automatically for the World Cup, with a 10th place available through an inter-continental play-off.Â
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