Vardy, 38, a free agent since leaving Leicester in May, put pen to paper on a one-year contract with an option to extend just hours before the transfer window closed, the Italian club said.
The player who scored 145 Premier League goals in 342 appearances is joining a team who have won their first two matches in Serie A, including a 2-1 win against AC Milan at the San Siro.
Vardy joins a growing contingent of former Premier League players in Serie A.
Most prominently, Scott McTominay has found success since leaving Manchester United and joining Napoil, helping the side from southern Italy to win the Serie A crown in May.
He has been joined at Napoli this season by former Manchester City star Kevin De Bruyne.