The midfielder has signed a three-year deal at the King Power Stadium to become the Foxes’ sixth signing of the summer.
Boss Claudio Ranieri has been chasing a new midfielder after failing to persuade Esteban Cambiasso to stay at the club and has now snapped up Switzerland international Inler.
Ranieri has already signed N’Golo Kante from Caen to beef up his midfield options and Inler joins for an undisclosed fee ahead of Saturday’s Barclays Premier League visit of Tottenham.
The Foxes have won both of their opening games under Ranieri, beating Sunderland and West Ham, to sit joint top of the table.
Ranieri had been tight-lipped over his transfer plans but did confirm he wanted another midfielder.
Asked about transfers last week, Ranieri said: “One more. A midfielder. I think one is enough. Of course, if two arrive then I’m happy.”
Inler joined Napoli from Udinese in 2011 and played for Switzerland and Turkey at Under-21 level before he opted to represent the Swiss and has won 84 caps.
Inler told Foxes Player: “I’m very happy and proud to play here because we’ve been in discussions for a long time. I’ve spoken with the manager and I’m very happy to be here because it’s a nice team. I’ve watched them lots of times and I like them.
“The manager was important for me, also the director Jon Rudkin and the owners. For me the whole package was very positive because they wanted me and now I’ll give everything.
“For me it was not so easy because I had many, many teams [chasing me] but in the end I decided with my heart and I came to Leicester City.”