Lukaku struck from a goalmouth scramble in the 38th minute to win the match for the top-ranked Belgians, who will start the Euros in confident mood after a convincing performance.
They shrugged off a bright spell at the start from Croatia to enjoy a comfortable victory, which might have had a more commanding scoreline had they taken all of their chances, as they confirmed their status as one the Euro 2020 favourites.
The goal came when fellow European Championship contenders Croatia failed to clear a corner and Lukaku was first to swivel and hook the ball home after Jason Denayer laid it on for him.
Lukaku had crashed a powerful strike against the crossbar seven minutes earlier as he shrugged off three markers to turn and fire goalwards. Yannick Carrasco
Lukaku should have had a second goal three minutes from the end but Croatia goalkeeper Dominik Livakovic made a goal-line stop at point-blank range to keep out his powerful header.
Captain Eden Hazard made his first appearance for Belgium since November 2019 as a substitute for the last eight minutes as he searches for fitness after an injury-plagued season.
It increases the forward’s chances of starting Saturday’s Group B match against Russia in St. Petersburg.
Croatia, runners-up at the World Cup three years ago, start their Group D campaign against England at Wembley next Sunday