Margaret Cavendish Bentinck, Duchess of Portland (11 February 1715 – 17 July 1785) was a British aristocrat, styled Lady Margaret Harley before 1734, Duchess of Portland from 1734 to her husband's death in 1761, and Dowager Duchess of Portland from 1761 until her own death in 1785.

The duchess was the richest woman in Great Britain of her time and had the largest natural history collection in the country, complete with its own curator, the parson-naturalist John Lightfoot, and the Swedish botanist Daniel Solander. Her collection included costly art objects such as the Portland Vase. Her ambition for her collection was for it to contain and to describe every living species.

She was a member of the Bluestockings, a group of social intellectuals led by women and founded by her great friend Elizabeth Montagu.