Blake Thomsen of The Cauldron first reported the news Thursday.
Donovan is the most decorated player in Major League Soccer history. The 34-year-old forward is the league's all-time leader in both goals (144) and assists (136) after stints with both the San Jose Earthquakes and the Galaxy.
He also owns both of those records for the United States national team with 57 goals and 58 assists at international level.
Donovan, whose career with the U.S. squad didn't receive a storybook ending after head coach Jurgen Klinsmann left him off the 2014 World Cup squad, walked away from the sport after winning the 2014 MLS Cup, his sixth league title overall, with Los Angeles.
There were some hints over the past few years that a return could happen, though.
Last June, he told Scott French of MLSSoccer.com he missed the feeling of competing on a weekly basis.
"I miss the game days," Donovan said. "When I'm here in the stadium on game days, I miss that. I don't miss the travel, I don't miss the hot summer training sessions, I don't miss having to take care of yourself 24-7, but the game days, the energy in the stadium, I certainly miss."
Those feelings clearly didn't subside because he explained to Gilbert Manzano of the Las Vegas Review-Journal in February that he was playing in the Global Legends Series in an attempt to satisfy that lingering competitive fire.
"I'm happy with my life right now, and maybe a game like this gives me the opportunity to compete a little bit and get the juices going again without being involved in a full way," Donovan said.
Apparently those efforts to keep playing without getting back involved with the professional grind weren't successful. So he's returning to the Galaxy.
Although it's unclear how long it will take Donovan to get back up to full speed and match fitness, it's a massive addition for the stretch run. The Galaxy sit third in the Western Conference with six matches to go as they cruise toward a playoff berth.
The California native is set to join an attacking group that's tied for the West lead in goals and includes Giovani Dos Santos, Robbie Keane and Steven Gerrard. His return also comes after fellow forward Gyasi Zardes suffered a broken foot that will keep him out for at least the rest of the regular season.