The 33-year-old leaves Napoli after two seasons in Italy, where he helped the club win the Serie A title in 2024-25. The deal is worth a reported 6 million euros fee which could rise through performance-related bonuses, according to Italian media.
Lukaku said he was delighted to join Fenerbahce and return to a country that holds special significance for him.
"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," he said.
"I will work hard every day, doing my best in training and on the pitch, trying to help the team in the best way possible. I'm 100% ready."
Lukaku's father, Roger, played for Genclerbirligi, giving the striker an early connection with Turkish football.
His arrival comes as Fenerbahce seek to challenge Galatasaray for domestic honours and secure Champions League football. They finished second in the Super Lig last season, three points behind their Istanbul rivals.
The club have strengthened significantly, bringing in Mason Greenwood, Nathan Ake, Vedat Muriqi, Marco Asensio, Matteo Guendouzi, N'Golo Kante, Milan Skriniar and Ederson.
Lukaku arrives with 316 goals in 681 senior club appearances and is Belgium's all-time leading scorer with 93 international goals.
An injury-hit 2025-26 campaign limited him to seven appearances for Napoli, but he remains a key figure for Belgium and scored three times as they reached the World Cup quarter-finals.
