Braun went 1 for 4 and stole a base in the fourth inning that helped set up a two-run double by Aramis Ramirez.
A smattering of boos during Braun's first at-bat was easily drowned out by the overwhelming applause. The former MVP was suspended for the final 65 games last year in the Biogenesis doping scandal.
Later, Braun had his infield single to lead off the sixth overturned to out after the call was challenged by Braves manager Fredi Gonzalez under Major League Baseball's new replay format.
The review took 58 seconds.
Yovani Gallardo (1-0) tossed six shutout innings for the win.
Making his first opening day start, Julio Teheran (0-1) allowed two runs and seven hits in six innings.
CARDINALS 1, REDS 0
CINCINNATI (AP) — Yadier Molina's homer broke a seventh-inning tie, leading St. Louis Cardinals to a victory over the Reds.
Adam Wainwright (1-0) used his refined sinker to finally get the best of the Reds, who have hit him like no other team. Wainwright allowed three hits in seven innings, fanning nine.
St. Louis escaped a threat in the eighth. Trevor Rosenthal retired all three batters in the ninth, finishing a three-hitter for the defending National League champions.
Bryan Price lost his managing debut with Cincinnati, which opened the season with eight players on the disabled list, its most since 2007.
Johnny Cueto (0-1) allowed three hits in seven innings.
PIRATES 1, CUBS 0, 10 INNINGS
PITTSBURGH (AP) — Neil Walker homered off Carlos Villanueva leading off the 10th inning, and the Pirates benefited from an overturned to call to beat the Cubs 1-0 on opening day.
Reliever Bryan Morris (1-0) won with the help of an overturned pickoff call in the top of the 10th under Major League Baseball's new replay system. Starter Francisco Liriano and four relievers combined for 11 strikeouts.
Emilio Bonifacio went 4 for 5 for Chicago, but the Cubs were 0 for 11 with runners in scoring position and wasted a fine start by Jeff Samardzija, who scattered five hits over seven innings.
MARLINS 10, ROCKIES 1
MIAMI (AP) — Jose Fernandez struck out nine and allowed one run in six innings, and Marlins newcomer Casey McGehee drove in four runs with two doubles to help Miami beat the Rockies.
The sellout crowd of 37,116 was the largest yet at Marlins Park, which opened two years ago.
The 21-year-old Fernandez earned the victory while becoming the youngest NL opening-day starter since Dwight Gooden in 1986, according to STATS.
Marcell Ozuna homered, doubled, singled and scored twice. Giancarlo Stanton had an RBI infield single and a run-scoring double. Adeiny Hechavarria singled three times and drove in a run.
Carlos Gonzalez homered for the Rockies in the sixth, but by then they trailed 6-0. Jorge De La Rosa (0-1) allowed five runs in 4 1-3 innings in his first opening day start.
NATIONALS 9, METS 7, 10 INNINGS
NEW YORK (AP) — Anthony Rendon hit a three-run homer in the 10th inning and drove in a career-high four runs, twice rallying the Nationals against the Mets' suspect bullpen for an opening-day victory.
Denard Span hit a tying double with two outs in the ninth off closer Bobby Parnell, and Ian Desmond put the Nationals in front for the first time with a sacrifice fly in the 10th. Rendon connected two batters later against former Washington pitcher John Lannan, securing a win for Matt Williams in his first game as a major league manager.
Stephen Strasburg struck out 10 over six innings in his third straight opening-day start.
Juan Lagares and Andrew Brown both homered for New York after they were surprise additions to the lineup.
Aaron Barrett (1-0) struck out two in a perfect inning to win his major league debut.
Jeurys Familia (0-1) took the loss, hurt by catcher Travis d'Arnaud's passed ball.
GIANTS 9, DIAMONDBACKS 8
PHOENIX (AP) — Buster Posey hit a two-run homer off new Arizona closer Addison Reed in the ninth inning and the Giants rallied from four runs down to beat the Diamondbacks.
Miguel Montero, who had three hits and reached base five times, homered to lead off the Arizona ninth and make it a one-run game.
Brandon Belt had three hits, including a solo home run, and singled ahead of Posey's homer.
Arizona scored four unearned runs off starter Madison Bumgarner in the fourth and led 7-3 after six. San Francisco got four runs with two outs in the seventh, with reliever Brad Ziegler walking in the tying run.
Jean Machi (1-0) pitched 1 2-3 innings of relief to get the victory. Reed (0-1), acquired from the Chicago White Sox in the off-season, got the loss.