Siegfried Loraine Sassoon CBE MC (8 September 1886 – 1 September 1967) was an English war poet, writer, and soldier. Decorated for bravery on the Western Front, he became one of the leading poets of the First World War. His poetry both described the horrors of the trenches and satirized the patriotic pretensions of those who, in Sassoon's view, were responsible for a jingoism-fuelled war. Sassoon became a focal point for dissent within the armed forces when he made a lone protest against the continuation of the war with his "Soldier's Declaration" of July 1917, which resulted in his being sent to the Craiglockhart War Hospital. During this period, Sassoon met and formed a friendship with Wilfred Owen, who was greatly influenced by him. Sassoon later won acclaim for his prose work, notably his three-volume, fictionalised autobiography, collectively known as the Sherston trilogy.

Siegfried Sassoon was born to a Jewish father and an Anglo-Catholic mother, and grew up in the neo-gothic mansion named Weirleigh (after its builder Harrison Weir) in Matfield, Kent. His father, Alfred Ezra Sassoon (1861–1895), son of Sassoon David Sassoon, was a member of the wealthy Baghdadi Jewish Sassoon merchant family. Siegfried's mother, Theresa, belonged to the Thornycroft family, sculptors responsible for many of the best-known statues in London; among them her brother, Sir Hamo Thornycroft.

There was no German ancestry in Sassoon's family; his mother named him Siegfried because of her love of Wagner's operas. His middle name, Loraine, was the surname of a clergyman she respected.

Siegfried was the second of three sons, the others being Michael and Hamo. When he was four years old his parents separated. During his father's weekly visits to the boys, Theresa locked herself in the drawing-room. In 1895, Alfred Sassoon died of tuberculosis.

Sassoon was educated at the New Beacon School, Sevenoaks, Kent; at Marlborough College, Wiltshire; and at Clare College, Cambridge, where from 1905 to 1907 he read history. He left Cambridge without a degree and spent the years after 1907 hunting, playing cricket and writing verse, some of which he published privately.

Although his father had been disinherited from the Sassoon fortune for marrying a woman who was not Jewish, Siegfried had a small private income that allowed him to live modestly without having to earn a living. Later, he was left a large legacy by an aunt, Rachel Beer, allowing him to buy the great estate of Heytesbury House in Wiltshire.

His first published success, "The Daffodil Murderer" (1913), was a parody of John Masefield's The Everlasting Mercy. Robert Graves, in Good-Bye to All That, describes it as a "parody of Masefield which, midway through, had forgotten to be a parody and turned into rather good Masefield."

Sassoon played for his village cricket team at a young age, and his brothers and three of his tutors were cricket enthusiasts. The Marchant family were neighbouring landowners, and Frank Marchant was captain of the county side between 1890 and 1897. Sassoon played for his house at Marlborough, once taking 7 wickets for 18 runs, and during this time he contributed three poems to Cricket magazine.