"After careful deliberation and after many conversations with those closest to me, I have decided that this will be my last season as a professional soccer player," Donovan wrote on his Facebook page.
The Galaxy confirmed his pending retirement plans on its website and planned a news conference later Thursday where Donovan's formal announcement was expected.
"We respect his decision to retire at the end of the season," Galaxy Coach Bruce Arena said in a statement. "Landon Donovan is one of the most significant figures in the history of soccer in the United States."
Donovan's decision came one day after he scored the winning goal in Major League Soccer's All-Star Game, where he made a record 14th consecutive appearance.
That's one of many records Donovan holds. He also has the career MLS scoring record with 138 goals and is the leading scorer in MLS playoff history with 22 goals.
He's also the all-time leader in goals (57) for the U.S. men's national team as well as in World Cup game appearances (12) and goals (five).
His decision also capped a bittersweet year for the 32-year-old native of Ontario, who began his MLS career with the San Jose Earthquakes in 2001.
After Donovan took a three-month sabbatical at the start of World Cup qualifying in 2013, the U.S. Coach Juergen Klinsmann cut Donovan from the team in May before the World Cup began, setting off a firestorm of debate over whether the move was warranted.
Regardless, Donovan said on his Facebook page that "I feel incredibly blessed and lucky to have played a role in the remarkable growth of MLS and US Soccer during my playing career."