Rivers will replace Jacoby Brissett as the Colts' starting quarterback next season. Brissett received a two-year contract from the Colts last September after franchise signal-caller Andrew Luck abruptly announced his retirement from the league.
Brissett guided the Colts to a 5-2 start last season, but a knee injury and accuracy issues caused the team to struggle toward the end of the 2019 campaign. Indianapolis lost seven of its final nine games and missed the postseason for the fourth time in the last five seasons.
Rivers will join a Colts team that returns one of the league's top offensive lines. He will also be helped by a rushing attack that finished seventh in the NFL last season.
The Chargers originally acquired Rivers in a 2004 draft-day trade with the New York Giants for Eli Manning. Rivers served as the Chargers' starting quarterback since the 2006 season before mutually parting ways with the franchise earlier this off-season.
Rivers struggled in his final season in Los Angeles, completing 66 percent of his passes for 4,615 yards and 23 touchdowns. He turned the ball over 23 times in 2019 after recording 24 turnovers in the previous two seasons combined.
Rivers, 38, leaves the Chargers as their franchise leader in 30 different categories, including passing yards (59,271) and touchdown passes (397).