Financial details were not disclosed but Italian media reported Fenerbahce agreed a €6 million ($6.92 million) fee plus performance-related bonuses for the former Chelsea, Manchester United and Inter Milan striker.
"It's an honour and a privilege to be joining your club, to join this club with a wonderful history," the 33-year-old said in a message to the Fenerbahce fans.
The veteran striker, who won the Serie A title with Napoli in 2024/25, endured a frustrating campaign last season that was hampered by injuries, limiting him to just seven appearances across all competitions.
However, Lukaku was still called up to the Belgium squad that reached the quarter-finals at the World Cup, where he scored three times.
"I want to thank the coach for the trust and the belief in me as a man, first of all, and also my qualities as a football player. I'm fit, 100% ready to go," Lukaku added.
"For me, it's also like a little emotional moment because after many years (I'm) going back to Turkey, where I lived as a young kid, where I discovered the professional world as a toddler with my dad," added Lukaku, whose father Roger played for Genclerbirligi.
"It's going to be special and a very emotional moment to be back there. I'm very proud that it is with Fenerbahce."
Fenerbahce finished second in the league last season, losing the title to Galatasaray by three points. They begin the new campaign against Genclerbirligi on Saturday.