The change, which was approved Tuesday night by the board of governors for the Academy of Motion Picture Arts and Sciences, takes effect at next year's Oscars on March 7.
The plan is a return to Oscar traditions of the 1930s and '40s, when a best-picture field of 10 or more films was common.
Academy President Sid Ganis said the board looked at last year's slate of films and decided there was room for more in the top category. "We nominated five, but there were many other great films last year," Ganis said.
Among last year's most acclaimed movies was the Batman blockbuster "The Dark Knight," which wound up snubbed. Another was "WALL-E," which won the Oscar for feature-length animation but was not among best-picture nominees.
Ganis said the broader field might make room for documentaries, foreign-language films, animated movies and even comedies, which typically do not fare well for best-picture nominations.
"Everybody says the academy will never nominate a comedy," Ganis said. "Well, maybe we will."
The Oscars have separate categories for animated, foreign-language and documentary films. A best-picture nomination would not preclude films from competing in those categories, as well.
With extra movies competing for the top honor, the Oscar show might draw a bigger TV audience as more fans tune to see how their favorite films fare, Ganis said.
The ratings for last February's Oscars were up solidly, but that followed a ceremony a year earlier that drew the lowest ratings ever. Along with other awards shows, the Oscars generally have been losing viewers over the past couple of decades.
Having 10 or more was common in Hollywood's golden age some 70 years ago. Ganis noted that 1939's 10 best-picture nominees were "Gone with the Wind," which won, "The Wizard of Oz," "Stagecoach," "Wuthering Heights," "Love Affair," "Goodbye, Mr. Chips," "Mr. Smith Goes to Washington," "Of Mice and Men," "Dark Victory" and "Ninotchka."
All are generally considered classics today.
The last time there were 10 nominees was the Oscar ceremony for 1943, when "Casablanca" won best picture. There have been only five nominees each year since then.
By David Germain