Markakis announced his decision to return Wednesday. The Braves' outfielder opted out of the season in July after teammate Freddie Freeman tested positive for the coronavirus.
He became the second Braves player to opt out at the time. More than a dozen MLB players have decided against playing this season.
"I've talked to my teammates, and I'd like to come back," Markakis said Wednesday. "Sitting at home and watching these guys compete the last couple of days and [seeing] all the risks they are taking by going out there. ... In the pit of my stomach. I felt like I needed to be out there.
"I had the opportunity to be welcomed back and here I am. I'm going to do my best to get back as quick as I can and help these guys out."
Freeman recovered from COVID-19 in time to start for the Braves opening day. Braves manager Brian Snitker said Wednesday he doesn't know when Markakis will return to the lineup.
Baseball officials granted Markakis' request for reinstatement, according to MLB.com.
Markakis, 36, hit .285 with nine home runs and 62 RBIs in 116 games last season for Atlanta. He hit .297 with 14 home runs and 93 RBIs in 2018 and was a first-time All-Star selection.
The Braves (3-3) won two out of three games against the New York Mets to start the season. Atlanta will attempt to tie its second series when the squad takes on the Tampa Bay Rays in the finale of a four-game meeting at 7:10 p.m. EDT Thursday at Truist Park in Atlanta.