Argentina could have clinched their berth for Qatar with victory against rivals Brazil but the game in San Juan ended 0-0.
However, Colombia, Uruguay and Chile all failed to win on Tuesday, meaning La Albiceleste are guaranteed a top four finish in CONMEBOL’s qualifying standings.
The qualification comes after Argentina’s Copa America triumph in July and amid a 27-game unbeaten run under boss Lionel Scaloni.
“This was a magnificent year,” Scaloni said. “Winning the Copa, qualifying for the World Cup unbeaten. It was a dream.
“We got four points against two very difficult rivals. We all like to win of course but these games help a team to mature. Without a doubt, the balance is positive.
“Having qualified so far in advance, in a really difficult qualifying section, is something we should be proud of.”
Ecuador are third in South American qualifying, with Tuesday’s 2-0 win in Chile boosting them six points clear of Colombia and Peru in fourth and fifth on 17 points.
Colombia had started qualifying strongly but drew 0-0 at home with Paraguay, meaning they are winless in their past five qualifiers.
Peru won 2-1 in Venezuela, while Bolivia moved into contention with a 3-0 home victory over Uruguay, who have lost four qualifiers in a row.