| Honorific Prefix: | The Right Honourable |
| The Lord Decies | |
| Honorific Suffix: | DL JP |
| Birth Name: | William Marcus de la Poer Horsley Beresford |
| Birth Date: | 12 January 1865 |
| Death Place: | Hornsey, London |
| Education: | Eton College |
| Alma Mater: | Christ Church, Oxford |
| Parents: | William Beresford, 3rd Baron Decies Catherine Anne Dent |
William Marcus de la Poer Horsley Beresford, 4th Baron Decies DL JP (12 January 1865 – 30 July 1910) was an Anglo-Irish aristocrat.
Beresford was born on 12 January 1865. He was the eldest of five sons born to William Horsley Beresford, 3rd Baron Decies, a Captain in the 10th Hussars and Grenadier Guards, and Catherine Anne Dent, daughter of Commander William Dent. His paternal grandparents were John Horsley-Beresford, 2nd Baron Decies and the former Charlotte Philadelphia Horsley (only daughter and heiress of Robert Horsley of Bolam House).
He was educated at Eton and Christ Church, Oxford.[1]
Upon his father's death on 3 July 1893, he succeeded as the 4th Baron Decies. He served as Deputy Lieutenant of Northumberland and Justice of the Peace for Northumberland.[2]
A "well-known and popular sportsman," he had an interest in hunting and "all outdoor sports." At the time of his death he had several horses in training with Pickering at Newmarket and with Maj. Morris at Wallingford.
On 12 March 1901, Lord Decies was married to Maria Gertrude Willoughby at St Michael's Church in Chester Square (in the Belgravia district of West London).[3] As Maria's father, Sir John Willoughby, 4th Baronet, had died in 1866, her brother, Maj. Sir John Willoughby, Bt of Jameson Raid fame, gave her away and William's groomsman was his brother, Capt. the Hon. John Graham Beresford, aide-de-camp to the Prince Arthur, Duke of Connaught.[4] [5] She was known for her "remarkable collection of cats."[1]
Lord Decies died, suddenly, on 30 July 1910, aged 45, at the Cottage Hospital in Hornsey, after having been "overcome by the oppressive heat" while attending the races at Alexandra Park.[6] As he died without issue,[7] he was succeeded in the barony by his younger brother, John.[8]