Hamels' deal does not contain any no-trade protection, a source told ESPN's Buster Olney.
Hamels joins a young Braves rotation that is fronted by Mike Soroka (22) and includes Max Fried (25) and Mike Foltynewicz (28). Two other members of the Braves' 2019 rotation are free agents -- right-hander Julio Teheran, after Atlanta declined his 2020 option, and left-hander Dallas Keuchel.
"How you quantify that is hard, but I think there is real value," general manager Alex Anthopoulos said Wednesday. "We're signing Cole first and foremost because we think he's going to help us win a lot of games ... and hopefully get to the World Series.
"No doubt in my mind ... I think Max Fried will get better just by seeing him and being around him. I think Soroka will get better."
Said Hamels: "I've always been following them in hopes of maybe having an opportunity to pitch with some of those guys. ... I think what I can provide might be beneficial."
When healthy, Hamels was as good as anyone in the National League last season, but he was plagued by injuries. Before an oblique strain sent him to the injured list, his ERA hovered under 3.00 for the Chicago Cubs. In 10 starts for the Cubs after returning, Hamels compiled a 5.79 ERA. He was never the same, mostly because of his mechanics.
"Trying to correct things while I built up the arm strength was a recipe for disaster," said Hamels, a four-time All-Star selection who will turn 36 on Dec. 27.
The 14-year veteran finished last season with a 7-7 record and 143 strikeouts with a 3.81 ERA. He threw fewer than 150 innings (141 1/3) for just the third time in his career, and his 1.39 WHIP was a career worst, largely due to walking 3.6 batters per nine innings, also a career high.
Hamels said the Braves, who have won two straight NL East division titles but failed to win a playoff series, "were one of six teams I was really following and really rooting for" in free agency.
"I reward the first team that actually does what they say they're doing," Hamels said. "Boom, we were there and it was going to happen."
Even after the Cubs were out of the playoff race, Hamels wanted to take the mound to show teams he was healthy.
"This is the first time I'm going into free agency, and I don't want that in the back of teams' heads, how I finished," he said.
When healthy, Hamels spotted his fastball exactly where he wanted to in 2019, setting up his devastating changeup as his out pitch. His cutter also proved to be a solid second weapon to help him generate more ground balls. Early on, he put together a string of 22 consecutive innings pitched without issuing a walk. But that control was lost after his injury -- he walked 21 in his final 42 innings in 2019.
He has never been given a qualifying offer, so the Braves have the option of doing that after the 2020 season and getting a draft pick if Hamels opts not to return in 2021.
Anthopoulos said he needed to be active because the Braves had a large number of expiring contracts.
"It hasn't been by design," Anthopoulos said. "We had a lot of free agents ... so we had a lot of work to do."
Anthopoulos said he's still looking for help at third base after Josh Donaldson became a free agent. He didn't rule out the possibility of adding another starting pitcher, and he said left-hander Sean Newcomb, who spent most of the 2019 season in the bullpen, will be given a chance to win a starting job.
Among other contenders for the final spot in the Braves rotation will be Kyle Wright, Bryse Wilson and Ian Anderson, one of the team's top prospects. The one-year deal with Hamels leaves room for Anderson to move into the rotation in 2021, if not before.
For his career, Hamels is 163-121 with a 3.42 ERA and 2,558 strikeouts, and he ranks seventh among active pitchers for wins. He was the MVP of the 2008 World Series and NL Championship Series for the Philadelphia Phillies, with whom he entered the majors in 2006 and spent the first nine-plus seasons of his career before being traded to the Texas Rangers in 2015.
ESPN's Jesse Rogers and Keith Law and The Associated Press contributed to this report.