Chesham (/ˈtʃɛʃəm/ CHESH-əm, locally /ˈtʃɛsəm/ CHESS-əm or /ˈtʃɛzəm/ CHEZ-əm) is a market town and civil parish in Buckinghamshire, England, United Kingdom, 11 miles (18 km) south-east of the county town of Aylesbury, about 26 miles (42 km) north-west of central London, and part of the London commuter belt. It is in the Chess Valley, surrounded by farmland. The earliest records of Chesham as a settlement are from the second half of the 10th century, although there is archaeological evidence of people in this area from around 8000 BC. Henry III granted a royal charter for a weekly market in 1257.

Chesham is known for its four Bs — boots, beer, brushes and Baptists. In the face of fierce competition from both home and abroad during the later 19th and early 20th centuries, the three traditional industries rapidly declined. The ready availability of skilled labour encouraged new industries to the town both before and after the Second World War. Today, employment in the town is provided mainly by small businesses engaged in light industry, technology and professional services.

From the early part of the 20th century, Chesham has experienced considerable expansion, with new housing developments and civic infrastructure. Chesham has become a commuter town with improved connection to London via the London Underground and road networks. The town centre has been progressively redeveloped since the 1960s and has been pedestrianised since the 1990s. The population at the 2021 census was 23,008.

There is archaeological evidence of the earliest settlement during the Late Mesolithic period around 5000 BC in East Street, Chesham where a large quantity of flint tools were found. The earliest farming evidence from the Neolithic era around 2500 BC. Bronze Age tribes settled in the valley around 1800 BC and they were succeeded by Iron Age Belgic people of the Catuvellauni tribe around 500 BC. Between 150 and 400 AD, there is evidence of Romano-British farming, and nearby at Latimer, there is archaeological evidence of a Roman villa and the planting of grapevines. However, the area was then deserted until the Saxon period around the 7th century.

Contrary to popular belief, the town is not named after the river; rather, the river is named after the town. The first recorded reference to Chesham is under the Old English name Cæstæleshamm, meaning "the river-meadow at the pile of stones" around 970 in the will of Lady Ælfgifu, who has been identified as the former wife of King Eadwig. She held an estate here which she bequeathed to Abingdon Abbey.

Prior to 1066 there were three adjacent estates which comprised Caestreham which are briefly recorded in the Domesday Book as being of 1+1⁄2, 4 and 8+1⁄2 hides, having four mills. The most important of these manors was held by Queen Edith, the widow of Edward the Confessor. Other land having been returned to the Crown it was in the hands of Harold Godwinson and his brother Leofwine Godwinson. Part of these later became Chesham Bois parish. After 1066 Edith kept her lands and William the Conqueror divided royal lands between his half-brother Odo, Bishop of Bayeux and Hugh de Bolbec.

The Domesday Book records that there were three manors in Cestreham and one at nearby Latimer. William the Conqueror shared out the estates between four of his dependants. The vast majority of land was granted to Hugh de Bolebec and smaller parcels to Odo, Bishop of Bayeux, Toustain Mantel and Alsi.

Before the 13th century, the three Cestreham manors were known as Chesham Higham, Chesham Bury and Chesham Bois. In the 14th century they were first recorded as 'the manors of Great Chesham'. Collectively they extended beyond the current Chesham town boundary. Together with the manor at Latimer they were held by the Earls of Oxford and Surrey. During the 16th century Greater Chesham was owned by the Seymour family who disposed of it to the Cavendish family who were the Earls and later Dukes of Devonshire. It is from the 15th century that the earliest surviving properties survive and are to be found close by the church in an area called the Nap, and along part of the present-day Church Street. Though gradually disposing of land, the Cavendishes maintained an influence in the town until the 19th century. The Lowndes family started purchasing land from the 16th century. William Lowndes was an influential politician and Secretary to the Treasury during the reigns of Mary II, William III and Queen Anne. He rebuilt the original Bury and manor house of Great Chesham in 1712. The Lowndes family settled in Chesham and over the next 200 years became equally influential both nationally through politics and the law and locally within the town as its principal benefactors.