The 33-year-old right-hander went 9-10 with a 3.24 ERA in 32 starts for the Kansas City Royals last season and should make a huge impact with the battered Braves, who are facing the prospect of losing Kris Medlen for the season and opening with Brandon Beachy and Mike Minor on the disabled list.
Medlen, who had been announced as Atlanta's opening-day starter, left a spring training outing Sunday after grabbing his right elbow. Initial tests showed ligament damage, and he will undergo further examination to determine if he needs surgery for the second time in less than four years. Medlen said he's already preparing himself to miss the entire season.
Beachy left a start Monday because of continuing problems with his right elbow, and Minor has yet to pitch this spring because of a sore shoulder.
''In light of what has happened over the past few days with our pitching staff, we felt it was incumbent on us to do everything we could to strengthen our starting pitching,'' Braves general manager Frank Wren told reporters before a game against Washington.
Santana will be pitching in the NL for the first time in his 10-year career. He spent eight seasons with the Los Angeles Angels before being traded to the Royals in 2013, helping Kansas City post an 86-76 record, their best since 1989.
But Santana, who made $13 million in the final year of his contract, turned down a $14.1 million qualifying offer from the Royals and became a free agent. Given his hefty contract demands, he remained unsigned as spring training began, despite putting up some impressive career numbers.
''This guy makes our rotation a lot better than where we are at now,'' Braves manager Fredi Gonzalez said, praising Wren. ''We lost Medlen and Beachy in a matter of 2 1/2 days and he went out there and got the best pitcher available for us.''
Santana threw a bullpen at the Braves spring training complex on Wednesday, which Gonzalez said went well. Despite his late entrance to camp, Santana could be ready for the start of the season.
''It will be close,'' Gonzalez said. ''We are kind of keeping our fingers crossed that we can get him enough innings where he can start for us.''
Santana was an AL All-Star in 2008, tossed a no-hitter in 2011, and has reached double-figure wins five times, going a career-best 17-10 in 2010 and twice winning 16 games.
Overall, he is 105-90 with a 4.19 ERA.
Santana's agent, Jay Alou, said he initially received a text from Wren on Sunday, then got another call Tuesday night while negotiating with other clubs.
It didn't take long to reach an agreement with Atlanta, a deal that includes award bonuses.
It's easy to see why the Braves felt a sense of desperation. Medlen went a team-leading 15-12 with a 3.11 ERA last season, while Minor was 13-9 with a 3.21 ERA. The team also was counting on a full recovery for Beachy, who was one of the NL's most effective pitchers in 2012 before he underwent season-ending elbow surgery.
Beachy has been limited to a total of 18 starts over the past two seasons, enduring several setbacks in his attempt to come back. After his latest problem, he will miss his next scheduled spring start and faces another trip to the DL.
Minor, who underwent urinary tract surgery on Dec. 31, hopes to pitch within the next week but also seems likely to start the season on the DL while he builds his arm strength.
Given all the health issues, the Braves got to the go-ahead from corporate owner Liberty Media to bump the payroll well above the projected $100 million limit. Wren called it ''an incredible decision by the organization.''
With opening day less than three weeks away, the only healthy members of the Braves' possible rotation are second-year pitchers Julio Teheran (14-8 last season) and Alex Wood, rookie David Hale and non-roster invitee Freddy Garcia. Hale and Garcia have both struggled this spring.
Atlanta also signed veteran Gavin Floyd, but he is coming off Tommy John surgery and won't be ready to pitch until at least May.
Now, add Santana to the list, though it will likely be mid-April before he is prepared to join the rotation. He has been throwing on the side and could make his spring training debut next week.
''We want to take the right amount of caution to get him ready right, and be ready for the long haul,'' Wren said.
By signing Santana, the Braves forfeited their first-round pick in June's amateur draft, No. 26 overall, and the Royals will receive a compensation-round selection, the 28th overall. Atlanta's first pick is now 32nd overall, as compensation for losing free agent catcher Brian McCann, who signed with the New York Yankees.
Santana's signing leaves only two free agents - Stephen Drew and Kendrys Morales - available from the 13 players who turned down qualifying offers. If they agree to one-year contracts after opening day, they wouldn't be subject to qualifying offers this November. If they wait until after the draft to reach agreements, clubs that sign them won't lose selections.
Among the 22 qualifying offers in the two years of the new labor contract, none have been accepted.
''Frankly, the players haven't adjusted to the change,'' New York Mets general manager Sandy Alderson said. ''To blame the system versus the marketplace and blame the system versus a misunderstanding of the marketplace I think is a little one-sided. From a management standpoint, I'd say, 'Hey, I'm surprised if you guys haven't taken a qualifying offer.' Maybe they should have.''