Markakis, the veteran outfielder who is in his sixth season with Atlanta, said Wednesday he changed his mind about sitting out after watching his teammates play the first five games.
"Sitting at home, watching these guys compete ... and all the risks they're going through going out there, in the pit of my stomach I felt I wanted to be out there," Markakis said.
He said he realized returning was possible after a phone call from Braves general manager Alex Anthopoulos earlier this week.
The 36-year-old Markakis says he has the full support of his family, including his three children, in his decision to return.
"It was nice to be home with the family, but more than anything, I know my family wanted me to be out there with [the Braves]," he said.
Markakis opted out July 6, when he said he was uneasy about playing the season without fans and then was swayed by his telephone conversation with teammate Freddie Freeman, who tested positive for COVID-19.
Freeman struggled with symptoms that included a high fever of 104.5 degrees, and Markakis acknowledged again Wednesday that he was influenced by Freeman's struggles.
Freeman returned for the start of the season.
Markakis said the call from Anthopoulos "made my decision a lot easier."
He said baseball "has been in my blood since I was 4 years old and it sucks ... it sucks watching baseball when you know you can go out and compete on a daily basis."
Markakis said he felt guilty about his decision to sit out the season and planned to apologize to his teammates.
"I bailed out on them, and when I see them I will apologize to them and hopefully be a part of them," Markakis said.
Markakis said he has worked out regularly but won't know when he will be ready to return to the Braves' roster until he talks with manager Brian Snitker.
Freeman, top setup man Will Smith, right-hander Touki Toussaint and infielder Pete Kozma tested positive for the coronavirus. Toussaint also has returned and Smith, who worked out in Atlanta during the team's opening road trip, could return this week.
The Braves opened their season without their top two catchers, Tyler Flowers and Travis d'Arnaud, after the two felt sick. Both had negative tests and rejoined the team for Wednesday night's home opener against the Tampa Bay Rays.
Also Wednesday, the Braves activated outfielder Scott Schebler to the 30-man roster. Catchers Alex Jackson and William Contreras and right-hander Chad Sobotka were optioned to the team's alternate training site at Triple-A Gwinnett.
Markakis, who is baseball's first player to return after opting out, hit .285 with nine homers and 62 RBIs in 2019. He was an All-Star in 2018.