The 28-year-old joined the Premier League side from Udinese in a much-hyped move in 2008 but soon fell out of favour with manager Rafael Benitez and barely played, losing his place in the Italy squad in the process.
"I'm happy to be able to wear the Napoli shirt," he told the website of the ambitious club, fourth in Serie A after a strong revival under new coach Walter Mazzarri. "I chose a prestigious club that has an important plan."
Benitez said Liverpool were interested in signing Atletico Madrid's Argentine midfielder Maxi Rodriguez and were continuing to fend off offers for Dutch forward Ryan Babel.
"It is true we've talked to Atletico Madrid (about Rodriguez) and we're interested in the player but still it's just conversations," the Spaniard told the club's website.
The 29-year-old, who joined Atletico in July 2005 after three seasons at Espanyol, has been out of favour since Quique Sanchez Flores took over from Abel Resino as coach at the end of October.
"It's a question of everything being more or less agreed, and now it's for Atletico and the player to talk and see what happens."
British media reports have suggested Premier League side Birmingham City have lodged a bid for Babel who has struggled for a starting berth under Benitez, but the Dutchman wants to stay at Anfield.
"We have received some offers but the player was clear the other day when he told me he wants to stay," Benitez said.
"At this moment everyone knows Ryan has the potential and he can improve."
One player who is poised to leave is Ukrainian international Andriy Voronin after the club said a transfer to Dynamo Moscow was "very close."
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