Lukaku’s brace, his second at this summer's tournament, means he is now the joint top scorer alongside Cristiano Ronaldo with four goals in two games.
Roberto Martinez’s Belgium side came into the Group G encounter having beaten Panama 3-0 in their opening match, a game that Lukaku stole the headlines in following his brace.
But it was Eden Hazard who took the early attention after he won, and then converted, a sixth-minute penalty.
After being fouled in the area by Syam Ben Youssef, the Chelsea forward fired past Tunisia goalkeeper Farouk Ben Mustapha to hand the advantage to Belgium and it took just 10 more minutes for Martinez’s men to double their lead.
This time it was Lukaku who scored after he was set up by Dries Mertens, but Tunisia got a goal back just two minutes later when Wahbi Khazri sent in a free-kick that Dylan Bronn headed past an outstretching Thibaut Courtois.
Tunisia felt they should have had a penalty when Khazri went down in the area after seemingly being hit in the face by Jan Vertonghen in the box.
But nothing was given, and Khazri proceeded to test Courtois with a fierce effort that the Chelsea goalkeeper saved.
Despite Tunisia’s threatening attacks, the team were shaky and nervous at the back, and Belgium took full advantage after Lukaku - who had missed a chance right in front of goal moments before - sent in Thomas Meunier’s through-ball on the stroke of half-time.
Tunisia, who were beaten 2-1 in added time by England in their first World Cup match, found themselves 4-1 down shortly after the break courtesy of a sensational goal from Hazard.
The forward chested a lofty ball down and, after sending it past the onrushing Ben Mustapha, finished off the move with coolness and precision.
With Belgium taking on England in their final group match next week, Martinez took off star men Lukaku and Hazard for Marouane Fellaini and Michy Batshuayi respectively.
Batshuayi could have added gloss to the scoreline but saw a shot cleared off the line, another hit the underside of the bar and another saved moments later by Ben Mustapha.
Batshayi finally grabbed his goal as the game entered stoppage time, latching on to fellow substitute Youri Tielemans' pass to slide in and beat the goalkeeper.
It was Batshuayi's fifth shot of the match, having come on in the 68th minute, more than any other player. Wahbi Khazri added a late consolation for Tunisia at the death.