Lukaku joined Chelsea for a second time in his career last August for a reported 97.5 million pounds ($118.3 million), penning a five-year deal.
But the 29-year-old failed to hold down a regular spot in coach Thomas Tuchel's team and returns to Inter to try to resurrect his career.
"I'm at the top of Inter HQ and look who I have here," Inter president Steven Zhang said in a post on Twitter, panning the camera to show Lukaku.
"I'm back," said Lukaku. "I'm very happy... Forza Inter!"
Lukaku was a player transformed in Milan after leaving Manchester United in August 2019. He changed his diet, lost weight and gained form as he scored 64 goals in 95 games in all competitions over two years.
His 24 league goals helped Inter clinch their first Serie A title in 11 years in 2020-21, ensuring he became a firm fan favourite.
Lukaku was seen as the missing piece in Tuchel's Chelsea setup, with the then European champions seemingly lacking a figurehead up front. But he made just 16 starts in the Premier League last term, scoring eight goals.
He did little to endear himself to Chelsea fans in December when he told Sky Italia that he was unhappy with Tuchel's system while also admitting he would like to return to Italy at some point in his career.