Leftwich, 28, signed for one year, ESPN said.
Batch broke his right collarbone in a 16-10 Steelers' win over Philadelphia Friday and is scheduled for surgery. Team officials said it has not been determined when Batch will be able to play again.
"We made an agreement with Byron Leftwich to sign and be a quarterback with the Pittsburgh Steelers," Tomlin said. "We have some familiarity there, because Ken Anderson, our quarterbacks coach, worked with him down in Jacksonville."
Leftwich has thrown for 9,321 yards and 52 touchdowns in 49 NFL games with Jacksonville and Atlanta.
The Steelers also signed 36-year-old punter Mitch Berger to a one-year deal. Tomlin said Berger and Paul Ernster will compete for the punter's position.