Minor league pitcher Todd Redmond was dealt to Cincinnati.
The Braves lost starting shortstop Andrelton Simmons to a broken right little finger last Sunday, and the rookie is likely to miss at least four weeks.
Janish, 29, has a .221 batting average with seven homers and 70 RBIs in 324 big league games.
“He’s a proven defensive talent,” Atlanta general manager Frank Wren said. “And we felt like, for our club, that was the most important aspect of the acquisition.”
Janish has a .981 fielding percentage in 283 career games at shortstop and is expected to start Sunday against the New York Mets.
Janish has spent the entire season at Triple-A Louisville, hitting .237 in 49 games at Louisville.
Wren said the Braves had a list of three or four guys they were scouting since Simmons’ injury. Atlanta manager Fredi Gonzalez indicated that rookie Tyler Pastornicky, who was recalled from Triple-A Richmond before Saturday’s game, was not a candidate to reclaim the job he lost due to weak defense on May 31.
“We were the pursuers, so you can’t necessarily dictate the timetable,” Gonzalez said. “We thought Janish was the best defensive shortstop out there that we could acquire. He was the number one guy on our list.”
Redmond, 27, is 69-56 with a 3.57 ERA in eight minor league seasons. The right-hander, who was assigned to Louisville, went 6-6 with a 3.58 ERA in 18 starts for Triple-A Gwinnett this season.
Reds general manager Walt Jocketty said the trade gives the Reds some pitching depth in the minors. The Reds have used only five starters through the first 87 games this season, a franchise record.
Cincinnati’s pitching staff has kept the Reds in contention in the NL Central. The staff ERA of 3.39 was Cincinnati’s best at the All-Star break since 1990.