Warner, 38, flanked by his family, cited his faith in God for helping him reach superior levels of success in professional football.
The former Northern Iowa product won two MVP Awards and played in three Super Bowls.
He started 15 games this season, passing for 4,583 yards, 30 touchdowns and 14 interceptions.
"It's been 12 unbelievable years, 12 of the best years of my life," Warner said at a news conference. "I'm excited about what's in front of me, and spending more time with my family."
The four-time Pro Bowl selection was sidelined by a concussion in the regular season and absorbed a hard hit from New Orleans' end Bobby McCray in a 45-14 loss to the Saints during the NFC Divisional Playoff game.
He retired with two years and $23 million left on his contract.
In his career, Warner completed 66.1 percent of his passes with 208 touchdowns, and 128 interceptions.