The Hollywood Reporter said Winfrey also contributed $1 million to the museum in 2007 and her combined $13 million donation makes her the institution's most generous benefactor.
To thank her for her support, the museum said it plans to name its 350-seat theater after her.
The institution is under construction and set to open near the Washington Monument in late 2015.
"I am so proud of African-American history and its contributions to our nation as a whole," Winfrey said. "I am deeply appreciative of those who paved the path for me and all who follow in their footsteps. By investing in this museum, I want to help ensure that we both honor and preserve our culture and history, so that the stories of who we are will live on for generations to come."
"We are inspired and profoundly grateful for Ms. Winfrey's generosity at this important time. Her gifts will forever be associated with harnessing the power of art and creative expression to build bridges between cultures and enrich people's lives," said the museum's founding director, Lonnie G. Bunch III.