Napoli have sold the 33-year-old for a fee reported at around €6-7 million, with roughly 40 per cent of that figure owed to Chelsea under the terms of Lukaku's 2024 departure from Stamford Bridge.
Napoli confirmed the transfer in a statement thanking the striker for his contribution.
"SSC Napoli announces it has permanently transferred the sporting services of Romelu Lukaku Bolingoli to Fenerbahçe Spor Kulübü. On 31 August 2024 Romelu made his debut in the blue shirt, scoring immediately in the comeback win against Parma at the Maradona.
"He made 45 appearances, registering 14 goals, including the unforgettable strike in the match that sealed the fourth Scudetto against Cagliari, capping a powerful surging run. With Napoli he won a league title and the Supercoppa Italiana.
"Good luck, Big Rom!" the club said.
Lukaku's exit brings to a close a spell that delivered major silverware but ended on a difficult note, with a serious muscle injury sustained in a pre-season friendly last year limiting his playing time and affecting his form heading into this summer's World Cup with Belgium.
He was unveiled by Fenerbahçe with a video message posted to the club's official channels.
"Hello dear Fenerbahçe fans. It is a great honour and privilege for me to join your club and be a part of such a magnificent club with such a glorious history.
"Sincerely, we have an excellent team and I am very happy to be part of such a strong squad. I will give my all every day, doing my best in training and on the pitch, trying to help the team in the most effective way possible. I want to make the most of every moment on the pitch to contribute to the team's success," Lukaku said.
He thanked the club's board and head coach for their trust and described the emotional significance of returning to Turkey, where his father Roger, also a professional footballer, played for Gençlerbirliği in Ankara during the 1996-97 season.
"I would like to thank the board of directors and our coach, firstly for the trust they have placed in me as a person, and secondly for their trust in my abilities as a footballer. I am 100 per cent ready, I am ready to go out onto the pitch.
"This is an emotional moment for me too, because I am returning to Turkey after so many years. As you know, I lived here as a child, and it was here that I discovered the world of professional football together with my father. So this return will be a truly special and emotional moment. I am very proud that this is happening with Fenerbahçe. Thank you for your support. See you soon."
Lukaku was due to land in Istanbul on Wednesday evening to complete the formalities of his move, bringing to an end weeks of speculation over his next destination following the confirmation that his Napoli career was over.