"I wanted you all to hear it from me 1st before u saw it on the news... I will be signing with the Washington Redskins," Garcon wrote on his Facebook page. "I'm very excited about the opportunity in front of me.
"I want to thank all Colts fans from the bottom of my heart for all the love and support you've shown me the past 4 years and I hope that at some level u will still follow my career, as I will continue to share my life with you on here."
Garcon's deal is a blockbuster: five years for $42.5 million with about $21.5 million guaranteed, according to FoxSports.com.
Garcon, 25, has 188 receptions for 2,519 yards and 16 touchdowns in his NFL career, all with the Colts.
Garcon wasn't the only receiver to agree to terms with the Redskins. NFL Network's Michael Lombardi reported Tuesday that the team also agreed with former 49ers receiver Josh Morgan on a two-year deal worth more than $6 million per season. Additionally, according to NFL Network insider Jason La Canfora, the Redskins retained defensive end Adam Carriker with a four-year deal worth $20 million.
The Associated Press contributed to this report.