36-year-old Smith gave an emotional farewell to the sport just minutes ago ... saying he's ready for a new challenge after making his improbable comeback.

"Two years ago I was stuck in a wheelchair, staring down at my mangled leg, wondering if I'd ever be able to go on a walk with my wife again, or play games with my kids in the yard," the 2020 Comeback Player of the Year said.

Smith credits getting a football back in his hand for his miraculous recovery ... adding, "I don't know what it was but all of a sudden I felt stronger, more driven and what once seemed impossible began to come into focus."

"I want to say thank you for believing in me and thank you for helping me believe in myself and the impossible," he added. "Because even though I have plenty of snaps left in me, after 16 years of giving this game everything I’ve got, I can’t wait to see what else is possible."

Smith says he will now spend time with his wife and kids ... and enjoy making up the time he's missed with them over the past 16 years.

Smith -- a superstar QB out of the University of Utah -- was the #1 overall pick to the San Francisco 49ers in 2005, where he played for 8 seasons.

He was traded to the Kansas City Chiefs in 2013 ... earning Pro Bowl honors 3 times in 5 seasons, and posting the league's best passer rating in 2017.

Smith joined the Washington Football Team in 2018 and was having a solid season before suffering a gruesome leg injury against the Houston Texans in Week 10.

After battling a flesh-eating bacteria and 17 operations, Smith worked nonstop to make his return to the field in 2020, appearing in 8 games for Washington.

"I was a skinny no-name recruit who wasn't supposed to play in college, let alone go to New York as a Heisman finalist or be the first one to have his name called on draft night," Smith added.