Chelsea striker Lukaku will play a key role at the upcoming FIFA World Cup for coach Marc Wilmots after fellow forward Christian Benteke was ruled out with a serious Achilles injury.
And he warmed up for the Brazil showpiece in style, as his well-taken treble highlighted Monday's win in Genk.
Januzaj's appearance was also noteworthy, with the Manchester United youngster – the subject of lengthy debate about his international future – eventually settling the matter after coming off the bench at half-time.
Another substitute, Tottenham's Nacer Chadli, scored the fourth before Kevin De Bruyne netted a late penalty as Belgium, eventually, won well.
This was by no means a vintage performance, though, and Wilmots will demand more from his side in further pre-World Cup friendlies against Sweden and Tunisia.
Belgium open their World Cup campaign against Algeria on June 17.
Goalkeeper Simon Mignolet sustained a minor injury in training on Saturday and Atletico Madrid's Thibaut Courtois was rested after featuring in the weekend's UEFA Champions League final.
With Hoffenheim's Koen Casteels also injured, 28-year-old shot-stopper Sammy Bossut – who plays his club football for Zulte-Waregem – made his international debut.
Lukaku gave Belgium the lead in just the third minute after Marouane Fellaini dispossessed Lars Gerson and played in the striker, who shot around advancing Luxembourg goalkeeper Anthony Moris and in.
Aurelien Joachim scored a stunning equaliser for Luxembourg just 10 minutes later, though, beating Bossut with an outstanding 30-yard strike.
Joachim held off the attentions of Thomas Vermaelen to take down a long ball, before lashing in with his left foot, and the same player caught Vincent Kompany napping in the 17th minute, only for Bossut to deny him with an impressive save.
Fellaini headed a corner over before Lukaku got his second with 23 minutes gone, as he pounced after Maxime Chanot's attempted clearance fell into his path, and scored, via a ricochet off Moris.
Lukaku lifted over Moris – but wide – and his search for a treble went on as the goalkeeper saved superbly to his right three minutes before the break.
Januzaj was one of Wilmots' five half-time changes and saw Lukaku become the first player to score a hat-trick for Belgium since Bob Peeters achieved the feat in 2001.
Lukaku gathered the ball outside the penalty area, cutting inside Tom Schnell and then beating Chris Philipps before netting a composed left-foot finish.
Mario Mutsch had a diving header saved at the other end before Chadli grabbed his third international goal 19 minutes from time with a driven effort from just inside the penalty area.
And there was still time for De Bruyne to make his mark, converting from the spot after another debutant Divick Origi was felled in the area.
