The news of Murphy's departure came Wednesday, a day after Ratner quit as producer of the Feb. 26 show.
Ratner left amid criticism of his use of a pejorative term for gay men in a question-and-answer session at a screening of his action comedy "Tower Heist," which opened last weekend and stars Murphy and Ben Stiller.
Murphy's exit deprives Oscar organizers of a top star for an often thankless job that's tough to fill, since some past hosts have found little to gain from the gig and plenty to lose if they do a poor job as emcee of Hollywood's biggest party.
The two sides departed with cordial words, though.
"I completely understand and support each party's decision with regard to a change of producers for this year's Academy Awards ceremony," Murphy said in a news release from the Academy of Motion Picture Arts and Sciences. "I was truly looking forward to being a part of the show that our production team and writers were just starting to develop, but I'm sure that the new production team and host will do an equally great job."
Academy President Tom Sherak bid Murphy farewell graciously.
"I appreciate how Eddie feels about losing his creative partner, Brett Ratner, and we all wish him well," Sherak said.
Still, losing Murphy is a blow to a ceremony that has struggled to pep up its image amid a general decline in its TV ratings over the last couple of decades and a rush of hipper awards shows that appeal to younger crowds, such as the MTV Movie Awards.
Oscar planners have sought to shorten the sometimes interminably long show and have tried new ways to present awards in hopes of livening things up.
They also have experimented with unexpected choices as hosts, which worked nicely with the song-and-dance talents of Hugh Jackman three years ago but backfired at last season's show, when perky Anne Hathaway was paired with lackluster co-host James Franco.
When the academy picked Murphy in September, it marked a return to the traditional funnyman as host, a formula that delivered some of the best-remembered Oscar pageants when Bob Hope, Johnny Carson and Billy Crystal ran the show.
Academy officials would not discuss the personnel changes or plans to replace Ratner and Murphy. Ratner's producing partner for the upcoming ceremony - Don Mischer, who co-produced last year's Oscars - remains on board for the show.
Any decision on who will replace Murphy as host likely will not come until Oscar planners make a decision on whether to bring in another producer to work with Mischer.
Organizers still have plenty of time. The show is more than three months away, and much of the work in staging it has to wait until Oscar nominations are announced Jan. 24, anyway.
There's also no great rush to name a new host. The academy did not announce Hathaway and Franco as hosts until the end of November last year, while the announcement on Jackman three years ago did not come until mid-December.