The Belgian moved from Inter to Chelsea last summer in a deal worth £97.5m.
But after only 16 Premier League starts last season, Lukaku is back at the San Siro.
Inter have paid a loan fee of around £7m to sign the 29-year-old.
"I'm back baby," Lukaku said in a video on Inter's Twitter page.
"I am very happy." Inter chief Stephen Zhang added: "The big boss is back. It is like deja vu, coming back here again."