The Jewel Tower is a 14th-century surviving element of the Palace of Westminster, in London, England. It was built between 1365 and 1366, under the direction of William of Sleaford and Henry de Yevele, to house the personal treasure of King Edward III. The original tower was a three-storey, crenellated stone building which occupied a secluded part of the palace and was protected by a moat linked to the River Thames. The ground floor featured elaborate sculpted vaulting, described by historian Jeremy Ashbee as "an architectural masterpiece". The tower continued to be used for storing the monarch's treasure and personal possessions until 1512, when a fire in the palace caused King Henry VIII to relocate his court to the nearby Palace of Whitehall.

At the end of the 16th century the House of Lords began to use the tower to store its parliamentary records, building a house alongside it for the use of the parliamentary clerk, and extensive improvements followed in 1621. The tower continued as the Lords' records office through the 18th century and several renovations were carried out to improve its fire-proofing and comfort, creating the present appearance of the tower. It was one of only four buildings to survive the burning of Parliament in 1834, after which the records were moved to the Victoria Tower, built for the purpose of storing archives, and part of the new neo-Gothic Palace of Westminster.

In 1869 the Jewel Tower was taken over by the newly formed Standard Weights and Measures Department, which used it for storing and testing official weights and measures. The tower became less and less suitable for this work as passing vehicular traffic increased, and by 1938 the department had given up on it in favor of other facilities. In 1948 the building was placed into the care of the Ministry of Works, which repaired the damage inflicted to the tower during the Second World War and restored the building extensively, clearing the surrounding area and opening the tower to tourists. Today the Jewel Tower is managed by English Heritage and receives about 30,000 visitors annually.

The Jewel Tower was built within the Palace of Westminster between 1365 and 1366, on the instructions of King Edward III, to hold his personal treasure. Edward had broadly three types of treasure: his ceremonial regalia, which was usually kept at the Tower of London or held by the Abbot of Westminster; the jewellery and plate belonging to the Crown, which was kept by the Royal Treasurer at Westminster Abbey; and his personal collection of jewels and plate. English monarchs during this period used their personal jewels and plate as a substitute for cash, drawing on them to fund their military campaigns, or giving them as symbolic political gifts. Edward accumulated what historian Jenny Stratford has described as a "vast store of jewels and plate", and his collection of personal treasure was at its greatest during the 1360s.

Edward had managed this last category of personal treasure through an organisation called the Privy Wardrobe. The Keeper of the Privy Wardrobe was responsible for guarding and recording the king's belongings, and dispatching particular items around the kingdom, potentially giving them as gifts to the monarch's family and friends. The Privy Wardrobe was initially based in the Tower of London in Edward's reign and became focused on handling the supplies for his campaigns in France. This probably encouraged the King to decide to build a new tower in Westminster to host a separate branch of the Privy Wardrobe specifically to manage his personal jewels and plate. In practice, this branch also managed the clothes, vestments and similar goods belonging to the royal household – effectively, the non-military parts of the King's property.

William of Sleaford was put in charge of the tower project as a whole; he was the clerk and surveyor of the king's works within the Palace of Westminster and the Tower of London, and became the Keeper of the Westminster Privy Wardrobe. The Jewel Tower was designed and built by Henry de Yevele, a prominent royal architect, supported by a team of masons he commissioned for both this project and a neighbouring piece of work to build a new clock tower nearby. Hugh Herland was taken on as the chief carpenter for both projects. The payments for the project were recorded on an 8-foot-6-inch (2.59 m) long parchment roll, which is now held in the Public Record Office.

Stone was brought in for the two towers: 98 boat-loads of rough stone and 13,782 feet (4,201 m) of dressed stone from Maidstone; 469 cart-loads from Reigate; 26 long tons (26 t) from Devon and 16 long tons (16 t) from Normandy. Timber was brought from Surrey, red floor tiles from Flanders and 97 square feet (9.0 m²) of glass purchased for the Jewel Tower alone. A contractor was employed to fix iron grilles to the windows, and 18 locks were purchased to secure the various doors. A main workforce of 19 stonemasons, up to 10 carpenters and other specialised tradesmen worked on the site, and in July 1366, a team of 23 labourers dug out the new moat over the course of a month.

The tower was constructed in the secluded south-west corner of the Palace of Westminster, overlooking the king's garden in the Privy Palace, the most private part of Westminster. The tower was positioned so as not to encroach on the existing palace, but this meant it was built on top of land owned by the neighbouring Westminster Abbey. It took six years for the abbey to convince the king to agree to compensate them for this annexation. William Usshborne, one of Edward's officials, was blamed for this and, when he later choked to death while eating a fish from a pond in the palace, the monks argued that this was divine justice for his role in the affair.