PHOENIX

Coach Bruce Arians once said he and quarterback Carson Palmer would ride away into the sunset together like a pair of old desert cowboys.

Well, they have.

Palmer announced his retirement after 15 NFL seasons on Tuesday in an open letter released by Cardinals, one day after Arians said he was leaving his long coaching career.

“Over the years, I’ve had teammates who decided to hang it up and I would ask them how they knew when it was time to walk away,” the letter began. “The answer was almost always the same: You just know. For me, it is time now. Why? Quite simply, I just know.”

Palmer, who turned 38 last week, made the announcement in an open letter released Tuesday by the Cardinals. Palmer missed the last nine games of what would be his final season with a broken left arm.

He called his long professional career “the most incredible experience of my life.”

Palmer and Arians were together the past five seasons, and they included some of the best days of the quarterback’s career.

Standing 6-foot-5 with a remarkably strong arm, Palmer was a Heisman Trophy winner at USC and the No. 1-overall pick by Cincinnati in 2002. He threw for 46,247 yards, 12th-most in NFL history, in a career with the Bengals, Oakland and Arizona.

“When I entered the league, I was a 23-year-old kid,” Palmer wrote. “I’m leaving a 38-year-old husband and father of four with memories and experiences that I will treasure for the rest of my life. And like most things in life, it feels like it all passed in a blink of an eye.”

Acquired by the Cardinals for only a sixth-round pick and a swap of seventh-rounders, Palmer’s strong arm was a great fit for Arians’ “no risk it, no biscuit” big-play passing game.

Palmer twice came back from torn ACL injuries. The first occurred in his playoff debut for Cincinnati in 2005. He was hurt on his first pass, a 66-yard completion. The second came in the sixth game of the 2014 season.

Palmer returned from that one to have probably the best year of his career. In 2015, he set single-season franchise records and career highs for yards passing (4,671) and touchdowns (35) while leading the Cardinals to a 13-3 record, second-best in the NFC. He won his only playoff game in four tries that season, in overtime over Green Bay.

“My family and I are beyond grateful for everything the game has given us as well as the love and support we’ve felt from fans everywhere we’ve been,” he said. “That’s been especially true in Arizona, where we never expected to end up but wound up being such a special place for us.”