Sir Adrian Cedric Boult, CH (/boʊlt/; 8 April 1889 – 22 February 1983) was a British conductor. Brought up in a prosperous mercantile family, he followed musical studies in England and at Leipzig, Germany, with early conducting work in London for the Royal Opera House and Sergei Diaghilev's ballet company. His first prominent post was conductor of the City of Birmingham Orchestra in 1924. When the British Broadcasting Corporation appointed him director of music in 1930, he established the BBC Symphony Orchestra and became its chief conductor. The orchestra set standards of excellence that were rivalled in Britain only by the London Philharmonic Orchestra (LPO), founded two years later.

Forced to leave the BBC in 1950 on reaching retirement age, Boult became principal conductor of the London Philharmonic Orchestra. The orchestra had declined from its peak of the 1930s, but under his guidance its fortunes were revived. He retired as its chief conductor in 1957, and later accepted the post of president. Although in the latter part of his career he worked with several other orchestras, including the London Symphony Orchestra, the Philharmonia Orchestra, the Royal Philharmonic Orchestra, and his former orchestra, the BBC Symphony, it was the LPO with which he was primarily associated, conducting it in concerts and recordings until 1978, in what was widely called his "Indian summer".

Boult was known for his championing of British music. He gave the first performance of his friend Gustav Holst's The Planets, and introduced new works by, among others, Elgar, Bliss, Britten, Delius, Rootham, Tippett, Vaughan Williams and Walton. In his BBC years, he introduced works by foreign composers, including Bartók, Berg, Stravinsky, Schoenberg and Webern. A modest man who disliked the limelight, Boult felt as comfortable in the recording studio as on the concert platform, making recordings throughout his career. From the mid-1960s until his retirement after his last sessions in 1978 he recorded extensively for EMI. In addition to several recordings that have remained in the active catalogue, Boult's legacy includes his influence on prominent conductors of later generations, including Sir Colin Davis and Vernon Handley.

Boult was born in Chester, Cheshire, the second child and only son of Cedric Randal Boult (1853–1950), and his wife, Katharine Florence (née Barman; d. 1927).

Cedric Boult was a Justice of the Peace and a successful businessman connected with Liverpool shipping and the oil trade; Cedric and his family had "a Liberal Unitarian outlook on public affairs" with a history of philanthropy.

When Boult was two years old the family moved to Blundellsands, where he was given a musical upbringing. From an early age he attended concerts in Liverpool, conducted mostly by Hans Richter. He was educated at Westminster School in London, where in his free time he attended concerts conducted by, among others, Sir Henry Wood, Claude Debussy, Arthur Nikisch, Fritz Steinbach, and Richard Strauss. His biographer, Michael Kennedy, writes, "Few schoolboys can have attended as many performances by great artists as Boult heard between 1901 and October 1908, when he went up to Christ Church, Oxford." While still a schoolboy, Boult met the composer Edward Elgar through Frank Schuster, a family friend.

At Christ Church college at Oxford, where he was an undergraduate from 1908 to 1912, Boult studied history but later switched to music, in which his mentor was the musical academic and conductor Hugh Allen. Among the musical friends he made at Oxford was Ralph Vaughan Williams, who became a lifelong friend.

In 1909, Boult presented a paper to an Oxford musical group, the Oriana Society, entitled Some Notes on Performance, in which he laid down three precepts for an ideal performance: observance of the composer's wishes, clarity through emphasis on balance and structure, and the effect of music made without apparent effort. These guiding principles lasted throughout his career. He was president of the University Musical Club for the year 1910, but his interests were not wholly confined to music: he was a keen rower, stroking his college boat at Henley, and all his life he remained a member of the Leander Club.