The 36-year-old has signed an 18-month contract, with Boca requesting additional time from the Argentine FA to register the forward for the Copa Libertadores round of 16, said local media.
"Happy to have you home," the Buenos Aires-based club wrote on X social media, formerly known as Twitter, along with a video welcoming the much-travelled player.
Cavani, who began his career with Uruguay's Danubio, moved to Europe in 2007 when he joined Palermo. After four seasons he switched to Napoli, where he scored 104 goals in 138 games and won the Coppa Italia.
In 2013, he signed for Paris Saint-Germain, where he played for seven years, becoming the club's all-time top scorer -- a record he held until last season when French forward Kylian Mbappé surpassed him by scoring 212 goals in all competitions.
He won six Ligue 1 titles, five French Cups and six League Cups in Paris plus a runners-up medal in the Champions League.
Cavani joined Manchester United in 2020 before moving to Valencia on a free transfer two years later. The Spanish club agreed earlier on Saturday to mutually terminate the striker's contract, which was due to expire next year.
Cavani has made 136 appearances and netted 58 goals for Uruguay to become their second-top scorer behind Luis Suarez. He helped his country win the 2011 Copa America title.