The Academy of Motion Picture Arts and Sciences confirmed his hiring later in the day.
"Some of the best moments of my career have happened on the Oscar stage. I am thrilled to be back there. Actually, I am doing the Oscars so the young woman in the pharmacy will stop asking my name when I pick up my prescriptions. Looking forward to the show," Crystal said via Twitter.
Crystal, who last emceed the Oscars in 2004, is replacing Eddie Murphy, who exited Wednesday as planned host of the next ceremony. Murphy quit one day after his "Tower Heist" director Brett Ratner bowed out as telecast producer.
Brian Grazer was announced Wednesday as Ratner's replacement.
Ratner, with whom Grazer worked on the big-screen comedy "Tower Heist," stepped down from producing the Oscars after he created a media firestorm by publicly using a slang word for gays.
Academy President Tom Sherak said he was happy Crystal has agreed to return as host.
"I'm thrilled to welcome Billy back to the Oscar stage," Sherak said in a statement. "He's a comic legend and Oscar icon, and it feels good to have him back where he belongs."
"Like so many others, I've been eager to see Billy host again. It's very gratifying that he agreed to do it with [producer] Don [Mischer] and me at the helm," Grazer added.
Only Bob Hope has served as host for more Academy Awards presentations than Crystal, with 19 ceremonies between 1940 and 1978.
Academy Awards for outstanding film achievements of 2011 are to be presented Feb. 26 at the Kodak Theatre in Hollywood, and televised live by the ABC Television Network.