The 28-year-old joined Liverpool from Udinese in 2008 for £7m but has made only 30 appearances, scoring two goals.
The left-back has also played 10 times for Italy but his last game came under former coach Roberto Donadoni in October 2007.
Dossena, who has struggled to become a regular under Rafael Benitez, has long been linked with a return to his homeland, with Napoli narrowly missing out in the summer.
Give your reaction to Dossena's move
"I am happy to be able to wear the Napoli shirt," Dossena told the Italian club's website.
"I have chosen a place of great prestige with big plans. I can't wait for the time to play at the San Paolo in front of an extraordinary public."
Dossena could make his debut on Sunday at home to Sampdoria.
Source: BBC Sport
Source: BBC Sport