Whittlesey (also Whittlesea) is a market town and civil parish in the Fenland district of Cambridgeshire, England. Whittlesey is 6 miles (10 km) east of Peterborough. The population of the parish was 16,058 at the 2011 Census.

Whittlesey appears in the Cartularium Saxonicum (973 CE) as 'Witlesig', in the 1086 Domesday Book as 'Witesie', and in the Inquisitio Eliensis. The meaning is "Wit(t)el's island", deriving from either Witil, "the name of a moneyer", or a diminutive of Witta, a personal name; + "eg", meaning "'island', also used of a piece of firm land in a fen."

Before the fens were drained, Whittlesey was an island of dry ground surrounded by them. Excavations of nearby Flag Fen indicate thriving local settlements as far back as 1000 BCE. At Must Farm quarry, a Bronze Age settlement is described as "Britain's Pompeii" due to its relatively good condition. In 2016 it was being excavated by the University of Cambridge's Cambridge Archaeological Unit. At Must Farm at least five homes of 3,000 years in age have been found, along with Britain's most complete prehistoric wooden wheel, dating back to the late Bronze Age.

Whittlesey was linked to Peterborough in the west and March in the east by the Roman Fen Causeway, probably built in the 1st century CE. Roman artefacts have been recovered at nearby Eldernell, and a Roman skeleton was discovered in the nearby village of Eastrea during construction of its village hall in 2010.

The town's two parishes of St Mary's and St Andrew's belonged to the abbeys in Thorney and Ely respectively until the Dissolution of the Monasteries about 1540. The two parishes were combined for administrative purposes by the Whittlesey Improvement Act of 1849. Despite the proximity of Peterborough, Whittlesey is in the Diocese of Ely.

Nearby Whittlesey Mere was a substantial lake surrounded by marsh until it was drained in 1851. According to the traveller Celia Fiennes, who saw it in 1697, the mere was "3-mile broad and six-mile long. In the midst is a little island where a great store of Wildfowle breed.... The ground is all wett and marshy but there are severall little Channells runs into it which by boats people go up to this place; when you enter the mouth of the Mer it looks formidable and its often very dangerous by reason of sudden winds that will rise like Hurricanes...." The town is still accessible by water, being connected to the River Nene by King's Dyke, which forms part of the Nene/Ouse Navigation. Moorings can be found at Ashline Lock, alongside the Manor Leisure Centre's cricket and football pitches.

Whittlesey was significant for its brickyards, around which the former hamlet of King's Dyke was based for much of the 20th century, although only one now remains, following the closure of the Saxon brickworks in 2011.

The local clay soil was also used to make cob boundary walls during a period in which there was a brick tax. Some examples of these roofed walls still stand today and are claimed to be unique in Fenland. Clay walls predate the introduction of brick tax in other parts of the country, and some were thatched.

Whittlesey was an important trade route in the late Bronze Age (about 1100-800 BCE). Evidence for this was found at the archaeological site of Must Farm, where log boats, roundhouses, bowls with food in them, and the most complete wooden wheel were housed.

In 1832, Whittlesey, then spelt Whittlesea, was ravaged by the second cholera epidemic, along with nearby Peterborough. According to a diary entry of Mrs Thomas Shaftoe Robertson, manageress of the Lincoln Theatre Circuit, "What a gap in my journal! April to November! But better not record such a summer as I have passed. God deliver me from such another. What suffering, what anguish, and loss! Whittlesea! Shall I ever have the idea of entering that place again? The cholera there raged in all its fury. I was numbered amongst its victims, and, false or true, was certainly dreadfully ill. All Peterborough was in a languishing state. Mr Walker, the surgeon, behaved most kindly, and never charged me a shilling." A year later this entry was amended: "Let me correct this error. The Lincoln Theatre Circuit also included at various times Whittlesey, Wisbech, Boston and other nearby towns.