The Norwegian international impressed in Italy during the second half of last season while on loan with Genoa, prompting the interest from Napoli.
Technical director David Weir said, “Leo’s had really successful loan spells in Germany, Italy and here in the UK in the Championship with Coventry City and Stoke City, so it was no surprise he attracted such interest from a top Italian club like Napoli.
“He is also keen to play regular senior football and establish himself as a permanent player. That was not something we were able to guarantee for Leo, so the transfer is good for both clubs and the player.
“It’s testament to the club’s academy, development programme and loan system, and equally to Leo. He has worked very hard and been an absolute professional in his approach and attitude.
“This transfer is a direct result of that dedication and professionalism on the training pitch, day to day and in matches. On behalf of the club I would like to wish him well in his future career and for the season ahead.”
Check out the latest Premier League transfer speculation below...
1. O’Brien deal back on
Lewis O’Brien and Harry Toffolo have agreed to sign for Nottingham Forest, despite reports on Tuesday suggesting that the double deal had collapsed. (Football Insider)
2. Wolves eye Benteke
Christian Benteke is a target for Wolves. The Crystal Palace striker has one year left of his current deal. (The Sun)
3. Gunners offered Akanji
Arsenal have been offered the opportunity to sign Borussia Dortmund central defender Manuel Akanji this summer. The German club are looking for a fee in the region of £21m. (Gazzetta Dello Sport)
4. Hammers in Lingard talks
West Ham are in negotiations with Jesse Lingard’s representatives with a growing feeling the player will opt for a return to the club this summer. (Sky Sports)