Ferdinand William Cavendish-Bentinck, 8th Duke of Portland (4 July 1889 – 13 December 1980) was a British peer and grandson of George Cavendish-Bentinck.
The son of Frederick W. Cavendish-Bentinck, grandson of George Cavendish-Bentinck, and great-grandson of Major General Lord Frederick Cavendish-Bentinck, fourth son of the 3rd Duke of Portland, the young Cavendish-Bentinck was educated at Eton and the Royal Military College, Sandhurst, passing out in 1910. He was commissioned into the 60th Rifles and was posted to Malta and British India before seeing active service in the European theatre of the First World War, which left him severely wounded. He then took up a posting as assistant adjutant at the Royal Military College. After the war, his main sphere of activity was in East Africa, where he served as Private Secretary to the Governor of Uganda (1925–1927), Chairman of the Agricultural Production and Settlement Board for Kenya (1939–1945), Timber Controller for East Africa (1940–1945), Member of the Government of Kenya for Agriculture and Natural Resources (1945–1955), and Speaker of the Kenya Legislative Council (1955–1960).
He was a Delegate to the Delhi Conference of 1940.
On the death of Lord (Francis) Morven Dallas Cavendish-Bentinck (1900 – 1950) he became the heir presumptive of his third cousin, William Cavendish-Bentinck, 7th Duke of Portland. However, on inheriting the dukedom and other titles in 1977, the new Duke was denied the great estates which for centuries had gone with the peerages, as the 6th Duke, before dying in 1943, had broken the entails and set up a trust ensuring that if his son the future 7th Duke left no son, the Welbeck Abbey estate and several others would go to his granddaughter Lady Anne Cavendish-Bentinck. He was able to do this as a result of the Law of Property Act 1925. The new Duke of Portland continued to live in Nairobi.
Elizabeth II shares an ancestor, in the 3rd Duke of Portland, through her maternal grandmother Cecilia Cavendish-Bentinck, and is thus a third cousin, once removed, of the 8th Duke of Portland.
In 1912, he married Wentworth Frances (d 1964), daughter of William James Hope-Johnston. They divorced in 1950 without issue.
In 1950, he married Gwyneth Ethel (d 1986), daughter of John Lesley Edwards and widow of Colonel David A.J. Bowie. Again, this marriage produced no issue.
He was succeeded by his younger brother, Victor Cavendish-Bentinck, as the 9th Duke upon his death.