Margaret Olivia Slocum (Mrs. Russell Sage) (1828-1918) was an American philanthropist. Upon the death of her husband she received a fortune estimated at more than $50,000,000, to be used as she saw fit. She was born at Syracuse, N. Y., and graduated from the Troy Female Seminary (later called the Emma Willard School) in 1847.

Her greatest single benefaction was the gift of $10,000,000 in 1907 to establish the Russell Sage Foundation. Three years later she gave $2,750,000 for the development of the Russell Sage Foundation Homes, a suburban community at Forest Hills Gardens, Long Island. In 1912, she acquired in the Gulf of Mexico and dedicated it as a home for wild birds. Up to 1915, the sum total of Mrs. Sage's gifts surpassed $23,000,000.