Samardzija (6-7) yielded one run and four hits in seven innings, rebounding from a rough June. The right-hander was 0-4 with a 12.27 ERA in his previous four starts, allowing a season-high nine runs during a 17-1 loss to the Mets on Wednesday.
''It's about time, you know?'' Samardzija said.
''I just thought I took my time from pitch to pitch. I thought I made the right pitch selection based on the previous pitch.''
Samardzija began the night with a 5.05 ERA. But he also was sharp in his first start against Atlanta this season, tossing seven solid innings in a 5-1 victory at Wrigley Field on May 7.
''Both times I've faced him, he's had unbelievable stuff,'' Atlanta pitcher Tommy Hanson. ''He throws hard and has that good split and starts throwing those sliders in there. Anytime you're mixing three pitches, it's tough.''
Cubs manager Dale Sveum said the key for Samardzija was using his slider, splitter and fastball, allowing ''a three-pitch pitcher to be a three-pitch pitcher.''
The splitter was the most devastating option.
''There were a couple times he threw it four or five times in a row,'' Braves manager Fredi Gonzalez said. ''And we had no chance on it. Especially left-handed hitters.''
Anthony Rizzo connected for Chicago, which has won four straight and six of seven. Rizzo, one of the Cubs' top prospects, has two homers in six games since he was recalled on Tuesday.
''The way we're playing right now is exciting,'' Samardzija said. ''It's solid all around.''
Samardzija said it may not be a coincidence that the Cubs' surge has come since Rizzo joined the team.
''It definitely hasn't hurt,'' Samardzija said. ''Rizzo's presence in our lineup has been big, a big left-handed hitter.''
Rizzo, who had two hits, is hitting .304.
Jeff Russell pitched the ninth for his second save.
Hanson (9-5) gave up four runs and seven hits in seven innings for his first loss since May 28. He went 4-0 with a 3.13 ERA in June.
Hanson made it through seven innings for only the sixth time in 17 starts, but appeared to fade in his final inning.
Bryan LaHair and Steve Clevenger started the seventh with singles, and Darwin Barney walked to load the bases. Valbuena, the No. 8 hitter who began the day with a .226 batting average, then doubled to right field for a 4-1 lead.
''I wanted to throw a back-door slider right there and it kind of leaked over the middle,'' Hanson said.
''I felt really good with my stuff. I was throwing strikes and getting ahead of guys. It just came down to that last inning.''
Rizzo hit Hanson's first pitch of the fourth inning over the wall in left-center for his second homer.
Atlanta tied it in the sixth. With two outs, Michael Bourn doubled to left field and scored on Martin Prado's single.
The Braves' only hit through five innings came with two outs in the second, when Dan Uggla, hustling all the way, beat out a grounder to shortstop Starlin Castro.
Andrelton Simmons led off the Atlanta third with a walk, took second on his first career stolen base and advanced to third on a wild pitch by Samardzija. Hanson then struck out and Bourn lined into a double play.
Barney made a diving stop on Simmons' ground ball behind second base and threw him out in the eighth. The second baseman then went the other way to snag a sharply hit grounder by pinch-hitter Eric Hinske for the second out.
NOTES: Cubs 3B Ian Stewart, who has been on the disabled list since June 12, will have surgery on his left wrist. Sveum said the date for the procedure has not been set. Stewart is batting .201 with five homers and 17 RBIs in 179 at-bats. ... Cubs RHP Ryan Dempster, who has been on the 15-day DL since June 18 with tightness in his right lat muscle, threw 40 pitches in the bullpen. Barring a setback, he plans to throw again in the bullpen on Wednesday. Sveum said he doesn't think Dempster will need a minor league rehab stint. ... Braves C Brian McCann, who was only 3 for 21 on the homestand before Monday night, was moved down to seventh in the batting order, the first time he has hit below sixth this season. McCann was 0 for 3 and is hitting only .224. ... The four-game series continues on Tuesday night with RHP Jair Jurrjens facing Cubs RHP Chris Volstad. ... Samardzija's previous high of nine strikeouts came on April 24 against St. Louis.