The striker, who scored 18 goals in 38 appearances for the Ligue 1 side in the 2019-20 season, moves to Naples in a deal reported to be worth €50 million (£45m/$59m).
The 10 times-capped Nigerian forward – who has scored four goals for his country – first moved to Europe in 2017 with Wolfsburg in the Bundesliga, before moving to Charleroi in Belgium, initially on loan and then in a permanent transfer.
He was hugely impressive in Belgium as he managed 36 goals in 37 appearances, earning him a €14m (£13m/$17m) move to Ligue 1, where his impressive scoring form continued.
Osimhen will spearhead Napoli’s attack next season as they look to return to the Champions League having missed out this year. The Neapolitan side will finish only seventh in Serie A, although they have guaranteed qualification for the Europa League group stage by winning the Coppa Italia.