Wilfred Trotter (1872-1939) was a British surgeon, a pioneer in neurosurgery. He was also known for his studies on social psychology, most notably for his concept of the herd instinct, which he first outlined in two published papers in 1908, and later in his famous popular work Instincts of the Herd in Peace and War. Trotter argued that gregariousness was an instinct, and studied beehives, flocks of sheep and a wolf packs.

He met Sigmund Freud several times. According to Ernest Jones (Freud's first biographer), "he was one of the first two or three in England to appreciate the significance of Freud's work, which I came to know through him. He was one of the rapidly diminishing group who attended the first International Congress at Salzburg in 1908."

Working at University College Hospital in London, as professor of surgery, he held the office of honorary surgeon to King George V from 1928 to 1932. He was also a member of the Council of the Royal Society that conferred their Honorary Membership on Professor Freud, whom he attended after his move to England. Later he was consulted about Freud's terminal cancer, in 1938.

Trotter was also the surgeon, at University College London for whom Wilfred Bion worked as a resident in his own medical training, before he famously studied groups and trained as a psychoanalyst at the Tavistock Institute. In her account of Bion's life 'The Days of our Years', his wife, Francesca writes of the great influence Trotter had on the direction of Bion's work on group relations.

Edward Bernays, author of Propaganda, and nephew to Freud, also refers to Trotter and Gustave Le Bon in his writings.