Nathaniel Mayer Victor Rothschild, 3rd Baron Rothschild, GBE, GM, FRS (31 October 1910 – 20 March 1990), was a British scientist, intelligence officer during World War II, and later a senior executive with Royal Dutch Shell and N M Rothschild & Sons, and an advisor to the Edward Heath and Margaret Thatcher governments of the UK. He was a member of the prominent Rothschild family.

Rothschild was born in Kensington, London, the only son of Charles Rothschild and Rózsika Rothschild (née Baroness Edle von Wertheimstein). Both parents were Jewish, his father a member of the Rothschild banking family and his mother the daughter of the first titled Jew in Austria. He grew up in Waddesdon Manor and Tring Park Mansion, among other family homes. He had three sisters, including Pannonica de Koenigswarter (who would become known as the "Jazz Baroness") and Dame Miriam Louisa Rothschild. His father died by suicide when Rothschild was 13 years old. He was educated at Stanmore Park preparatory school (which he later dubbed a "hell hole") and Harrow School, where the combination of archaic privileges and pointless rituals served only to annoy and bore him.

At Trinity College, Cambridge, Rothschild read physiology, French, and English, and was considered impressive enough an undergraduate to be spared the rigours of sitting the Natural Sciences Tripos, thus allowing him to embark immediately on a career in scientific research. Working in the Zoology Department, he was awarded a fellowship by Trinity in 1935 and a PhD two years later. He played first-class cricket for the University and Northamptonshire, where his experience of batting against the Nottinghamshire pair of Harold Larwood and Bill Voce he was later to describe as the most alarming of his life. At Cambridge he was known for his playboy lifestyle, driving a Bugatti and collecting art and rare books.

Rothschild joined the Cambridge Apostles, a secret society, which at that time was predominantly Marxist, though he stated himself that he "was mildly left-wing but never a Marxist". He became friends with Guy Burgess, Anthony Blunt and Kim Philby, members of the Cambridge Spy Ring. His flat in London was shared with Burgess and Blunt; this later aroused suspicion that he was the so-called Fifth Man in the Spy Ring. In 1933, Rothschild gave Blunt £100 to purchase "Eliezer and Rebecca" by Nicolas Poussin. The painting was sold by Blunt's executors in 1985 for £100,000 and is now in the Fitzwilliam Museum.

Rothschild inherited his title at the age of 26 following the death of his uncle Walter Rothschild, 2nd Baron Rothschild on 27 August 1937. He sat as a peer in the House of Lords, but spoke only twice there during his life (both speeches were in 1946, one about the pasteurization of milk, and another about the situation in Palestine). In November 1945 he joined the Labour Party.

Rothschild was recruited to work for MI5 during World War II in roles including bomb disposal, disinformation and espionage, winning the George Medal for "dangerous work in hazardous circumstances". He was the head of B1C, the "explosives and sabotage section", and worked on identifying where Britain's war effort was vulnerable to sabotage and counter German sabotage attempts. This included personally dismantling examples of German booby traps and disguised explosives.

With his assistant Theresa Clay, he ran the "Fifth Column" operation, which saw MI5 officer Eric Roberts masquerade as the Gestapo's man in London in order to identify hundreds of Nazi sympathizers.

In Who Paid the Piper? (1999), an account of CIA propaganda during the Cold War, author Frances Stonor Saunders alleges that Rothschild channelled funds to Encounter, an intellectual magazine founded in 1953 to support the "non-Stalinist left" to advance US foreign policy goals.