Donovan put pen to paper on a one-year contract with the Liga MX side to mark the second time he has come out of retirement. The 35-year-old has been out of action since 2016, when he rejoined the LA Galaxy less than two years after helping the team win the MLS Cup in 2014.
His unexpected return to the pitch follows days of speculation, which was triggered by Leon's social media account joking about signing Donovan as part of a marketing ploy to reveal the club's third jersey.
Before the Mexican side posted a photo of Donovan wearing a Leon shirt, club president Jesus Martinez said: "I knew it, you always wanted to use green. Welcome to #LaFiera! You are already a player of @clubleonfc."
Donovan responded by declaring his love for the city, and appeared to take a shot at President Donald Trump's plan to build a wall along the border between the United States and Mexico.
"I loved the city. You have the best fans @clubleonfc is a historic and winning team. I do not believe in walls, I want to go to Mexico, wear green and win trophies with Leon. See you soon!"
Donovan spent the majority of his career in Major League Soccer, where he won six league championships. He began his career with German outfit Bayer Leverkusen before representing the San Jose Earthquakes and Galaxy. He also enjoyed loan spells with Bayern Munich and Premier League club Everton.