Westfield is a city located in Hampden County within the Pioneer Valley of western Massachusetts, United States. It lies at the western edge of the downfaulted Connecticut River Valley where the Westfield River emerges from the Berkshire Hills and flows through the center of the city toward the Connecticut River. The municipality is part of the Springfield Metropolitan Statistical Area and serves as a significant urban center on the fringe of the greater Northeast megalopolis, bordered by towns such as Southampton, Holyoke, and Agawam.

The area was originally inhabited by the Pocomtuc people and known as Waranoke. European trading houses were established in 1639, and Massachusetts asserted jurisdiction after a boundary survey. Land was incrementally purchased from Native Americans and granted to English settlers beginning in 1658, with permanent settlement occurring in the 1660s. Westfield was incorporated as an independent town in 1669 and re-incorporated as a city in 1920, having been the westernmost settlement in the Massachusetts Colony until 1725.

The city is split into South Side and North Side by the Westfield River, with the northwestern section known as Wyben. Zoning laws prohibit buildings taller than three stories because no structure may exceed the height of the town's firetruck ladders following a major fire in 1952 that destroyed the six-story Westfield Professional Building. The landscape features linear cliffs of volcanic trap rock known as East Mountain and Provin Mountain, which are traversed by the Metacomet-Monadnock Trail.

Westfield is historically known as the Whip City due to its prominence in the buggy whip industry during the 19th century. The city was also at the center of the Pure Food movement, with resident Louis B. Allyn serving as a pure foods expert for McClure's Magazine. In 1939, Westfield became the first city in Massachusetts and New England to elect a female mayor when Alice Burke defeated the incumbent. The downtown business district caters increasingly to students from Westfield State University.

The population was 40,834 at the 2020 census, with students comprising some 15% of the total. The economy has shifted from manufacturing to warehousing and commercial uses, supported by convenient access to interstate highways. The city is located 10 miles west of Springfield and 95 miles west of Boston. It lies in a floodplain zone and experiences severe flood episodes due to its large, steep upstream watershed, despite systems of pumps and dikes.