The 28-year-old had been a key player for the Naples-based club since joining from Danubio in 2007.
However, the Uruguay international believes that the time is right for him to embark on a "new adventure".
"I'm happy to be here," he told Sky Sport Italia. "I'm joining a great club. They wanted me here and I'll do my best to repay Inter's faith in me.
"Am I afraid of the competition for a place in the team? No, I'm never scared.
"The Napoli fans will always remain in my heart. It was beautiful to play for Napoli, but I'm now starting a new adventure with Inter and I hope to do well."
Inter are believed to have paid £1m to make the loan deal possible.