The Manchester United striker has now scored more than any other Belgian at a World Cup, having netted four goals in two games so far.
He has also scored more goals (seven) than any other Belgium international over World Cups and European Championships combined.
Lukaku is also the first player since Argentina great Diego Maradona in 1986 to score two goals or more in consecutive World Cup matches.
The 25-year-old moves level with Cristiano Ronaldo in the race for the Golden Boot at this year's tournament, having kicked off with two goals in Belgium's opening-game 3-0 win over Panama.
The former Chelsea man added two more in the first half of the meeting with Tunisia on matchday two for Roberto Martinez's side, calmly slotting home on both occasions to take his tally to four for the tournament.
In what has been a hugely productive 2017-18 campaign, Lukaku has followed up his 27 goals in all competitions for United with seven goals for Belgium in June alone.
Already Belgium's all-time top goalscorer, Lukaku's record now stands at 40 goals in 71 caps for his country.