St James's Palace is the most senior royal palace in London, England. The palace gives its name to the Court of St James's, which is the monarch's royal court, and is located in the City of Westminster. Although no longer the principal residence of the monarch, it is the ceremonial meeting place of the Accession Council, the office of the Marshal of the Diplomatic Corps, and the London residence of several members of the royal family.

Built by order of King Henry VIII in the 1530s on the site of an isolated leper hospital dedicated to Saint James the Less, the palace was secondary in importance to the Palace of Whitehall for most Tudor and Stuart monarchs. Initially surrounded by a deer park and gardens, it was generally used as a hunting lodge and as a retreat from the formal court and occasionally as a royal guest house. After the Palace of Whitehall burned down, the palace took on administrative functions for the monarchy. It increased in importance during the reigns of the Hanoverian monarchs but began to be displaced by Buckingham Palace in the early 19th century. After decades of being used increasingly only for formal occasions, the move was formalised by Queen Victoria in 1837.

The palace now houses a number of offices and official societies and collections. All ambassadors and high commissioners to the United Kingdom are still accredited to the Court of St James's. The palace's Chapel Royal is still used for functions of the British royal family. Official receptions are also regularly hosted at the palace.

The palace was built mainly between 1531 and 1536 in red brick, and its architecture is primarily Tudor in style. The Queen's Chapel was added in the 1620s, and Clarence House was built on palace grounds directly next to the Palace in the 1820s. A fire in 1809 destroyed parts of the palace, including the monarch's private apartments, which were never replaced. Some 17th-century interiors survive, but most were remodelled in the 19th century.

The palace was commissioned by Henry VIII on the remote site of a former leper hospital dedicated to Saint James the Less. He first used it as a hunting lodge for his newly enclosed deer park, St. James's Park. The new palace, secondary in the king's interest to Whitehall Palace, was constructed between 1531 and 1536 as a smaller residence to escape formal court life.

Much smaller than the nearby Whitehall, St James's is arranged around four courtyards: the Colour Court, the Ambassador's Court, the Engine Court and the Friary Court. These remain enclosed except for Friary Court, which in modern times is only enclosed by apartments on three sides, the fourth being open to Marlborough Road to accommodate public gatherings. The most recognisable feature of the palace is the north gatehouse; constructed with four storeys, the gatehouse has two crenellated flanking octagonal towers at its corners, and a central clock dominating the uppermost floor and gable; the clock is a later addition and dates from 1731, refurbished 1834. The palace is decorated with the initials H.A. for Henry and his second wife, Anne Boleyn. Henry had the palace constructed in red brick, with detail picked out in darker brick.

The palace was remodelled in 1544, with ceilings painted by Hans Holbein and was described as a "pleasant royal house". Two of Henry VIII's children died at Saint James's, Henry FitzRoy, 1st Duke of Richmond and Somerset, and Mary I. Elizabeth I often resided at the palace, and is said to have spent the night there while waiting for the Spanish Armada to sail up the Channel.

Prince Henry, Prince of Wales, the eldest son of King James and Anne of Denmark, lived at St James's Palace until his death in 1612. The gardens were improved for him by Alphonsus Fowle. A riding school, one of the first in England, was built for Henry at St James's Palace between 1607 and 1609, and then a library with sculptural decoration by Maximilian Colt. Henry also installed a menagerie with pet birds including a pair of ostriches.