The Barnes Foundation in Philadelphia, best known for its French modernist paintings, also houses one of the earliestcollections of African art in the US. Its founder, Albert C. Barnes, regarded African sculpture as the purest example of three-dimensional form, acquiring over 100 works during an intense three-year period leading up to the opening of the Foundation in 1925. Most of the African art was acquired from the Paris dealer Paul Guillaume; who also introduced Barnes to the work of Amedeo Modigliani, Giorgio de Chirico and many others.
You can browse the collection here.
Further reading: Representing Africa in American Art Museums -A Century of Collecting and Display, edited by Kathleen Bickford Berzock and Christa Clarke, which features an essay on The Barnes Foundation & Paul Guillaume and African art, by Solveig Pigearias and Michèle Hornn, Tribal Art Magazine, no. 59, Spring 2011: pp. 78-91.