Lavezzi, 27, is PSG's first recruit this summer as they look to build a competitive team for next season's Champions League after finishing second in Ligue 1.
"He is a very very important player," general manager Leonardo told a news conference at the Parc des Princes.
"He will mark the club's history."
Lavezzi, one of the crowd favourites at San Paolo, hopes he will find the same connection with the demanding PSG fans.
"I will do what I can to enjoy the same relationship with the Paris crowd," he said.
No financial details on the deal were disclosed but media reports suggested the transfer fee was about 30 million euros.
Lavezzi joined Napoli from San Lorenzo in 2007 and scored nine goals in 30 Serie A appearances last season. He has 16 caps for Argentina and three goals.
Reuters
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