John Wells, the "E.R." creator, will direct the darkly comic movie based on Tracy Letts' play.
The casting has been a long time coming: Published reports as far back as 2010 had the two Oscar-winners in talks for the picture.
"August: Osage County" is the story of the women of the Weston family, who have grown apart from one another, but return to their house in the Midwest after a family crisis.
Streep will play Violet Weston, the dysfunctional matriarch. Roberts will play her adult daughter, Barbara.
“After seeing Meryl Streep's mesmerizing portrayal as Margaret Thatcher in 'The Iron Lady,' it has me even more excited and proud to co star with her," Roberts said in a statement.