The 29-year-old joined Chelsea from Inter Milan for a club record £97.5m at the start of last season but could not live up to expectations.
Lukaku posted only eight Premier League goals; a massive drop from the 24 he managed for Inter Milan in Serie A the previous season.
Lukaku's performances tailed off after an impressive start and he found himself out of Thomas Tuchel's starting eleven on a number of occasions.
On the positive side, the former Manchester United and Everton striker still finished as Chelsea's top scorer with 15 goals from 44 appearances across all competitions.
Inter will pay a loan fee of £6.8m for Lukaku, who helped the Nerazurri win the Serie A title in the 2020/2021 season.
He told the club's official website: "It's like coming home. My family and I were so happy here thanks to the people, the fans and my team-mates. From the first day I arrived here, everyone helped me settle in.
"I'm very happy. I kept my home here when I went to England, so that means something. I'm pleased to be back and now I can't wait to get out there with my team-mates."
"It's an amazing feeling. What we achieved here a year ago was fantastic. That goes for the team, the fans and also me personally.
"Inter have given me so much and I hope to do even better than last time. It's now time to work hard as a squad and hopefully do even better than before."
Lukaku has been given the number 90 jersey at Inter.
