Her fellow celebrity judges will be Ben Folds of Ben Folds Five and Shawn Stockman of Boyz II Men.
The announcement regarding Bareilles' involvement was made by Paul Telegdy, executive vice president of alternative programming for NBC and Universal Media Studios.
"I love the fact that on this show, the judges are each so rooted in the world they preside over," Telegdy said in a statement. "Simply put, they can do what the contestants are doing and that means a lot. Sara will bring a sublime technical knowledge and huge appreciation for 'The Sing-Off ' -- she's a fan and we're thrilled she's coming on board."
"I'm extremely excited to be joining the panel of judges for 'The Sing-Off' this fall!" added Bareilles. "A cappella music is very near and dear to my heart, and I have been a huge fan of the show from the beginning. I love the opportunity to be a part of something that celebrates a genre of music that I just adore and believe in. I can't wait for it to start."
Season 3 of "The Sing-Off" is to premiere Sept. 19, with recording artist Nick Lachey returning as host.