A 2-1 win away to Croatia ensures unbeaten Belgium will finish top of Group A and they became the third European team to secure a place in Brazil after Netherlands and Italy.
Later on Friday, Germany, Switzerland and Russia can also seal automatic qualification from the penultimate round of qualifying matches across Europe.
Victory on a wet night in Zagreb prompted joyous celebrations from the Belgian players in front of a sizeable and vociferous contingent of travelling fans.
"I'm super happy for the team. We always believed we could qualify and now it's here," Lukaku told Belgian television.
The pacey striker had only previously played 52 minutes of qualifying but, starting in place of the injured Christian Benteke, notched his first goals of the campaign for a nation who boast one of the most talented squads in Europe.
Lukaku broke the deadlock from Steven Defour's defence-splitting pass and doubled the lead seven minutes before the break, racing away from halfway before lifting the ball over prone keeper Stipe Pletikosa and walking the ball into the net.
He was brought off after 68 minutes, embracing coach Marc Wilmots as he left the rain-soaked pitch.
Croatia second-half substitute Niko Kranjcar grabbed a late consolation goal for the hosts, who have 17 points and are assured of a runners-up finish but were jeered by their fans after a lacklustre performance.
Bulgaria's hopes of securing second place behind Italy in Group B, and a potential playoff match, suffered a huge setback after they had two players sent off in a 2-1 defeat in Armenia with the hosts netting the winner three minutes from time.
The Bulgarians had Nikolay Bodurov dismissed on the stroke of halftime and went down to nine men after 63 minutes when Svetoslav Dyakov was shown a second yellow card.
Yura Movsisyan won it for Armenia when he broke clear to score late on.