A report from ESPN's Leon Lecanda on Wednesday said the deal would be for two years, with the Mexico City side announcing on Thursday that the El Tri star had officially joined the team.
The Liga MX Clausura season kicks off on Jan. 7. Las Aguilas finished atop the regular season table of the recently completed Apetura campaign but were ousted in the playoffs, with Atlas winning the title.
Dos Santos was a free agent after the Galaxy stated in late November that it had parted ways with the former designated player.
"I feel I'm in pretty good shape. I am 31 years old. People say that I am coming to retire here; that's not true. I still have a lot to give in my career and to this club. I am hungry. I want to win titles with América and make the fans happy," Dos Santos said.
During his time with the Galaxy that started in 2017, Dos Santos made 103 appearances for the club and was selected as an MLS All-Star in the 2018 and 2019 seasons. In his last two years, injuries took a toll on regular appearances.
The move to Liga MX is a first for Dos Santos, who has previously also played in LaLiga for Barcelona and Villarreal. Jonathan's older brother Giovani, currently a free agent, previously played for Club America before being dropped over the summer.
"Since I was a kid, I dreamed that one day I would play in America. My father played here, Gio too, the Dos Santos' already made history in this club," Dos Santos said. "My dad always told us, you have to play for America, you have to fulfill that dream. It's the best team in Mexico. I left the country 20 years ago, so having the opportunity to return and play for the best team is a big honor for me."