The grant announced Thursday will support the capital campaign for design and construction of the new museum.
Foundation President Allan Golston says the museum will make the stories and history of African Americans available to everyone.
Groundbreaking is expected in 2012, and the opening is slated for 2015. Construction is expected to cost about $500 million, with half the funds to be provided by Congress.
Museum director Lonnie Bunch has been quietly working to raise funds. He says the Gates Foundation grant is a vote of confidence for the project.