The Philharmonia Orchestra is a British orchestra based in London. It was founded in 1945 by Walter Legge, a classical music record producer for EMI. Among the conductors who worked with the orchestra in its early years were Richard Strauss, Wilhelm Furtwängler and Arturo Toscanini; of the Philharmonia's younger conductors, the most important to its development was Herbert von Karajan who, though never formally chief conductor, was closely associated with the orchestra in the late 1940s and early 1950s. The Philharmonia became widely regarded as the finest of London's five symphony orchestras in its first two decades.

From the late 1950s to the early 1970s the orchestra's chief conductor was Otto Klemperer, with whom the orchestra gave many concerts and made numerous recordings of the core orchestral repertoire. During Klemperer's tenure Legge, citing the difficulty of maintaining the orchestra's high standards, attempted to disband it in 1964, but the players, backed by Klemperer, formed themselves into a self-governing ensemble as the New Philharmonia Orchestra. After thirteen years under this title, they negotiated the rights to revert to the original name.

In Klemperer's last years the orchestra suffered a decline, both financial and artistic, but recovered under his successor, Riccardo Muti, who revitalised the orchestra in his ten-year term from 1972 to 1982. The orchestra's standards remained high throughout the controversial chief conductorship of Giuseppe Sinopoli from 1984 to 1994, and the more orthodox tenure of Christoph von Dohnányi between 1997 and 2008. Esa-Pekka Salonen, principal conductor from 2008 to 2021 was succeeded by Santtu-Matias Rouvali.

The Philharmonia has had many celebrated players in its ranks and has commissioned more than 100 compositions. It gives more than 160 concerts a year, tours widely, and from its inception has been known for its many recordings.

The name "Philharmonia" was adopted by the impresario and recording producer Walter Legge for a string quartet he brought together in 1941, comprising Henry Holst, Jean Pougnet, Frederick Riddle and Anthony Pini. The name was taken from the title page of the published score Legge used for the first work they recorded. Temporarily augmented to a septet, the ensemble gave its first concert in the Wigmore Hall, the main item being Ravel's Introduction and Allegro. With several changes of personnel the quartet continued to play in concert and in the recording studio during the Second World War. In 1942 the editor of The Gramophone, Compton Mackenzie, wrote that he had no hesitation in calling the Philharmonia the best string quartet in the country.

During the war, Legge was in charge of the music division of the Entertainments National Service Association (ENSA), which provided entertainment for British and allied armed forces. In this role he was in close touch with many first-rate musicians in the armed services, from whom he intended to draw when creating a new orchestra after the end of the war. He later set out his guiding principles:

Before the war, Legge had been assistant to Sir Thomas Beecham at the Royal Opera House, Covent Garden. Both men wrongly assumed that they would be able to resume their control of the opera house after the war, and Legge conceived of a new orchestra based there, operating on the lines of the Vienna Philharmonic – playing in the pit for the opera and also giving concerts and making records on its own account. The committee appointed by the British government to re-establish opera and ballet at Covent Garden abandoned the pre-war system of opera seasons, in favour of a permanent year-round company. Neither Beecham nor Legge was invited to run it. Legge nevertheless decided to go ahead with his plans to form an orchestra. Although London already had three permanent symphony orchestras – the London Symphony Orchestra (LSO), BBC Symphony Orchestra (BBC SO) and London Philharmonic (LPO), their personnel and standards had declined during the war and he was convinced he could do better.

Legge secured the services of many talented young musicians still serving in the armed forces. He first assembled a "Philharmonia String Orchestra" for recordings in 1945, composed of musicians from the RAF orchestra. He then recruited wind and percussion players, including some of the country's top instrumentalists who had been playing in other orchestras during the war. At the Philharmonia Orchestra's first concert, on 27 October 1945, more than sixty per cent of the players were still officially in the services. Beecham conducted the concert (for the fee of one cigar), but as he refused to be Legge's employee and Legge refused to cede control of the orchestra, they went their separate ways. Beecham founded the Royal Philharmonic Orchestra (RPO) the following year.