Cameroon won by the same score on aggregate after the teams drew 0-0 in Tunisia. The team follows Nigeria and Ivory Coast, who won their playoffs Saturday, in sealing a place at next year's tournament in Brazil.
Pierre Webo scored after four minutes for Cameroon and Benjamin Moundandjo made it 2-0 in the 30th at Omnisports Ahmadou-Ahidjo stadium. Ahmed Akaichi pulled one back for the visiting Tunisians just after halftime, but Jean Makoun added a third in the 65th and scored again in the 86th to seal Cameroon's progress.
"Brazil, here we come," Cameroon captain Samuel Eto'o, who reversed his decision to retire from internationals to play in this playoff, told the crowd through a public address system after the game.
Cameroon, Africa's first quarter-finalist in 1990, will play in its seventh World Cup, a major boost for the nation after it failed to qualify for the last two African Cups.
"We've been through several long and difficult years," Makoun said. "Now, I think we identified the problem, which is concord among us. We can achieve better results in the future and I think our performance today shows that we have a bright future."
If Ghana and Algeria win their playoffs on Tuesday, as expected, the continent will be represented by the same five nations that played at the 2010 World Cup in South Africa.
Webo slammed home a left-footed shot in front of around 60,000 in Yaounde for an immediate lead for Cameroon. The home team was also denied a penalty for what appeared to be a clear handball before Moundandjo made it 2-0 after cutting in from the right flank to score.
After being brought on by Tunisia coach Ruud Krol, Akaichi struck after a defensive mix-up by the Cameroonians for 2-1 straight after the break.
But Cameroon powered back as Makoun headed home a Benoit Assou-Ekotto corner and grabbed a second in the final five minutes after a shot rebounded off the post.