Ryu lasted only five innings but was dominant as he gave up only one hit, an infield single by Adam Duvall in the second.
Duvall homered in the seventh off Thomas Hatch.
Anthony Bass issued a one-out walk in the ninth to Marcell Ozuna, who moved to second on Travis d'Arnaud's single to left field. Duvall hit into a double play to end the game as Bass earned his second save.
Ryu (1-1) struck out eight as he showed the form that produced a majors-leading 2.32 ERA for the Dodgers in 2019. The left-hander from South Korea had an 8.00 ERA in his first two starts for Toronto.
''I wish this win would have come a lot earlier,'' Ryu said through a translator. ''I guess the third time is the charm. I hope this sets me up for future success.''
Ryu signed a four-year, $80 million contract with the Blue Jays in December. The strong start helped the Blue Jays end a three-game losing streak.
''That's the guy we were expecting, changing speeds, keeping hitters off balance,'' Toronto manager Charlie Montoyo said. ''That's a great sign for us, that he pitched like that, and a boost for us.''
The Braves were forced to recast their lineup after placing Ozzie Albies and Matt Adams on the injured list and giving Freddie Freeman a day off.
The new-look lineup, which included Charlie Culberson at second base and Austin Riley at first base, didn't resemble the unit which had 13 hits in a 10-1 win over the Blue Jays on Tuesday night.
Toronto took a 1-0 lead in the second when Vladimir Guerrero Jr. doubled just beyond Ender Inciarte's leaping attempt at a catch at the center-field wall. Guerrero scored from third on Danny Jansen's fly ball to right field.
Bo Bichette singled, stole second and scored on Cavan Biggio's single in the fifth. The run-scoring hit by Biggio knocked Sean Newcomb (0-1) out of the game after the left-hander allowed five hits and two runs in 4 2/3 innings.
Left-hander Ryan Borucki struck out Freeman, who was pinch-hitting for Riley, after Duvall's homer. Right-hander Jordan Romano struck out Ronald Acuna Jr. to cap a perfect eighth.
Braves manager Brian Snitker saw good signs in Newcomb's start. The Braves need the left-hander to solidify a spot in a rotation which this week lost ace Mike Soroka to a season-ending torn right Achilles tendon.
''That was encouraging tonight from Newcomb,'' Snitker said, adding the start was ''a lot better'' than his first two outings.
''We kept the game right there,'' he said. ''We just couldn't get a big hit.''
Following Thursday night's final game of the three-game series in Atlanta and three games in Boston, the Blue Jays will finally play their first game at their temporary 2020 home in Buffalo, New York, on Tuesday against Miami. Modifications are being made to the field and clubhouses.
Right-hander Nate Pearson, who made a brief stop at Triple-A Buffalo, already can provide a scouting report: Sahlen Field is a pitcher's park.
''It's pretty cool,'' Pearson said. ''I like pitching there. I think it's a pretty fair ballpark.
''I really liked the mound. I thought it was nice and tall. I felt like I got on the hitters pretty well. I felt like it's a pitcher's park instead of a hitter's park.''
Blue Jays: OF Derek Fisher (left quad strain) was placed on the 10-day IL and OF Billy McKinney was recalled before the game. ... INF Travis Shaw, who was activated on Tuesday after being placed on family medical leave on July 27, was given another day out of the lineup to take batting practice and possibly be available as a pinch-hitter. ... Montoyo said ''there's a good chance'' RHP Chase Anderson (oblique) will pitch in relief, at least initially, when he returns from the IL.
Braves: Albies (right wrist bone bruise) and Adams (left hamstring strain) were placed on the 10-day injured list. Albies will be shut down for a week in hopes he won't miss more than 10 days. OF Nick Markakis was reinstated from the restricted list and grounded out to first base as a pinch-hitter in the eighth. C Alex Jackson was recalled from the team's alternate training site.
After throwing five scoreless innings against Washington in his debut on July 29, Pearson will try again for his first win in his second major league start when he faces Atlanta's Touki Toussaint (0-0) in Thursday night's final game of the three-game series.
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