Sir Giles Gilbert Scott (9 November 1880 – 8 February 1960) was a British architect known for his work on the New Bodleian Library, Cambridge University Library, Lady Margaret Hall, Oxford, Battersea Power Station, Liverpool Cathedral, and designing the iconic red telephone box.

Scott came from a family of architects. His father George Gilbert Scott Jr. was a co-founder of Watts & Co., which Scott became the second chairman of. He was noted for his blending of Gothic tradition with modernism, making what might otherwise have been functionally designed buildings into popular landmarks.

Born in Hampstead, London, Scott was one of six children and the third son of George Gilbert Scott Jr. and his wife, Ellen King Samson. His father was an architect who had co-founded the architecture and interior design company Watts & Co. in 1874. His paternal grandfather was Sir (George) Gilbert Scott, a more famous architect, known for designing the Albert Memorial and the Midland Grand Hotel at St Pancras Station.

When Scott was three, his father was declared to be of unsound mind and was temporarily confined to the Bethlem Royal Hospital. Consequently, his sons saw little of him. Giles later said that he remembered seeing his father only twice. A bequest from an uncle in 1889 gave the young Scott ownership of Hollis Street Farm, near Ninfield, Sussex, with a life tenancy to his mother. During the week Ellen Scott and her three sons lived in a flat in Battersea, spending weekends and holidays at the farm. She regularly took them on cycling trips to sketch buildings in the area, and encouraged them to take an interest in architecture. Among the buildings the young Scott drew were Battle Abbey, Brede Place and Etchingham Church; Scott's son, Richard Gilbert Scott, suggests that the last, with its solid central tower, "was perhaps the germ of Liverpool Cathedral".

Scott and his brothers were raised as Roman Catholics; their father was a Catholic convert. Giles attended Beaumont College on the recommendation of his father who admired the buildings of its preparatory school, the work of J. F. Bentley. In January 1899 Scott became an articled pupil in the office of Temple Moore, who had studied with Scott's father. From Moore, or Ellen Scott, or from his father's former assistant P. B. Freeman, Scott got to know the work of his father. In a 2005 study of Scott's work, John Thomas observes that Scott senior's "important church of St Agnes, Kennington (1874–77; 1880s–93) clearly influenced Giles's early work, including Liverpool Cathedral Lady Chapel."

In later years Scott remarked to John Betjeman, "I always think that my father was a genius. … He was a far better architect than my grandfather and yet look at the reputations of the two men!" Scott's father and his grandfather had been exponents of High Victorian Gothic; Scott, when still a young man, saw the possibility of designing in Gothic without the profusion of detail that marked their work. He had an unusually free hand in working out his ideas, as Moore generally worked at home, leaving Freeman to run the office.

In 1901, while Scott was still a pupil in Moore's practice, the diocese of Liverpool announced a competition to select the architect of a new cathedral. Two well-known architects were appointed as assessors for an open competition for architects wishing to be considered. G. F. Bodley was a leading exponent of the Gothic revival style, and a former pupil and relative by marriage of Scott's grandfather. R. Norman Shaw was an eclectic architect, having begun in the Gothic style, and later favouring what his biographer Andrew Saint calls "full-blooded classical or imperial architecture". Architects were invited by public advertisement to submit portfolios of their work for consideration by Bodley and Shaw. From these, the two assessors selected a first shortlist of architects to be invited to prepare drawings for the new building.

For architects, the competition was an important event; not only was it for one of the largest building projects of its time, but it was only the third opportunity to build an Anglican cathedral in England since the Reformation in the 16th century (St Paul's Cathedral being the first, rebuilt from scratch after the Great Fire of London in 1666, and Truro Cathedral being the second, begun in the 19th century). The competition attracted 103 entries, from architects including Temple Moore, Charles Rennie Mackintosh and Charles Reilly. With Moore's approval, Scott submitted his own entry, on which he worked in his spare time.