Terms of the deal were not released, but multiple reports peg it as a one-year, $25 million pact.
"Philip is one of the most decorated quarterbacks in the NFL and we are fortunate to add an experienced player of his caliber to our organization," Colts general manager Chris Ballard said in a team statement. "His familiarity with our coaching staff and offensive system in addition to his experience were attractive qualities during our evaluation process. Philip is a fierce competitor and his veteran leadership will be crucial in the continued development of our young roster."
Rivers, 38, spent all 16 seasons with the Chargers, where he worked with head coach Frank Reich and offensive coordinator Nick Sirianni in the 2013-15 seasons and already has a feel for the Colts' offense.
"I'm sure there are certainly some things that have changed - I look forward to learning those and getting a feel for those - but for the most part it is going to be almost the same system that we've ran since I've been in since '13," Rivers said during a conference call with reporters. "In a lot of ways you find out over a long career that a lot of us all run the same thing. You call it a little differently. ...
"I know what those guys are trying to get out of a play and why they are calling this. They understand what I think and how I look for things. There is a good dynamic there from the way we communicate. I think that is a positive and I do think there was a trust factor that was built in our time here in San Diego."
An eight-time Pro Bowl selection, Rivers is sixth all-time in career passing yards (59,271) and touchdowns (397). With the Chargers, he completed 64.7 percent of his 7,591 pass attempts.
Rivers said he considered retirement but wanted to continue his career because he felt he could still play at a high level.
"There really wasn't necessarily a plan like, 'Hopefully we can do one year, hopefully we can do two.' It was just kind of, 'Hey, whatever works out best for both sides,' and it happened to work out this way," Rivers said. "And, again, like I said, we're a ways away. We know how this league is, things change in a hurry, but certainly hope that it's longer than that.
"But I think it's just important to focus on this season and focus on right now. How can I get better and help the Colts today? And doing that throughout the rest of this offseason and throughout this season."
Rivers was the No. 4 overall pick in the 2004 NFL Draft by the New York Giants, and shipped to the Chargers for No. 1 overall pick Eli Manning, who said he wouldn't play in San Diego. He left the Chargers as the holder of more than 30 franchise records.
The Chargers opted not to re-sign him after his play declined last season. He passed for 4,615 yards but his touchdown passes dropped from 32 in 2018 to 23, and his interception count rose from 12 to 20.
With the Colts, he is set to replace Jacoby Brissett, who became the starter after the abrupt retirement of Andrew Luck before the start of the 2019 season.
Brissett was 7-8 as a starter in 2019, throwing for 2,942 yards with 18 touchdowns and six interceptions.
Also on Saturday, the Colts released quarterback Brian Hoyer.
Hoyer started his career with the Patriots as an undrafted free agent from Michigan State, then had stops with the Arizona Cardinals, Cleveland Browns, Houston Texans, Chicago Bears and San Francisco 49ers before returning to the Patriots in 2017. He played in four games (one start) with the Colts last season and passed for 372 yards, four touchdowns and four interceptions.
For his career, Hoyer has completed 59.1 percent of his passes for 10,274 yards with 52 touchdowns and 34 interceptions.
--Field Level Media