Terms of the deal were not disclosed.
Van Noy is the latest defensive addition for the Chargers, who have beefed up that unit this offseason, trading for edge rusher Khalil Mack and also signing cornerback J.C. Jackson and defensive tackle Sebastian Joseph-Day in free agency.
The New England Patriots released Van Noy in March, ending his second tenure with the franchise, to clear about $4.1 million in salary-cap space.
The 6-foot-3, 250-pound Van Noy played in 16 regular-season games last season for New England, totaling 66 tackles to rank fourth on the team. He also had five sacks (second among Patriots players) and 10 passes defended in addition to a forced fumble and interception return for a touchdown.
Van Noy, 31, had signed a two-year, $12 million contract with the Patriots in March 2021 (with $6 million guaranteed) after he was released by the Miami Dolphins.
Van Noy was part of two Super Bowl championship teams from 2016 to 2019 while with the Patriots before signing with the Dolphins in 2020.
Originally a fourth-round draft pick by the Detroit Lions in 2014, Van Noy has 28.5 sacks in eight seasons.
ESPN's Mike Reiss contributed to this report.