"SSC Napoli are delighted to announce the signing of Victor Osimhen from Lille on a permanent transfer," the Italian club said in a statement.
Napoli president Aurelio De Laurentiis told 'La Gazzetta dello Sport' the deal is for a club record fee of 70 million euros plus another 10 million in bonuses.
"We had been following Victor Osimhen for some time..Gattuso (Gennaro, coach) and Giuntoli (Cristiano, sporting director) wanted him. They convinced me to make the investment of 70 million euros plus another 10 million bonuses. If we also consider wages, this transfer exceeds 100 million".
Sources had told ESPN that Napoli had made an offer of €70 million in June and were confident of landing the 21-year-old, whose desire had also been to play in the Premier League. Manchester United and Chelsea were keen on Osimhen.
Nigeria international Osimhen only joined Lille last summer from Charleroi for just €12m but had an excellent season in France with 13 league goals.
Osimhen previously spent two seasons at Bundesliga side VfL Wolfsburg, where he made his debut in 2017, before moving to Charleroi.
Osimhen has scored four goals in eight appearances for Nigeria. He also won the Under-17 World Cup in 2015, finishing as the tournament's top scorer with 10 goals.
As well as Osimhen, Napoli are interested in another Lille player, Brazilian defender Gabriel, with a bid of €30m on the table, according to ESPN sources.
Information from Reuters was used in this report.