A costermonger, coster, or costard is a street seller of fruit and vegetables in British towns. The term is derived from the words costard (a medieval variety of apple) and monger (seller), and later came to be used to describe hawkers in general. Some historians have pointed out that a hierarchy existed within the costermonger class and that while costermongers sold from a handcart or animal-drawn cart, mere hawkers carried their wares in a basket.

Costermongers met a need for rapid food distribution from the wholesale markets (e.g., in London: Smithfield for meat, Spitalfields for fruit and vegetables or Billingsgate for fish) by providing retail sales at locations that were convenient for the labouring classes. Costermongers used a variety of devices to transport and display produce: a cart might be stationary at a market stall; a mobile (horse-drawn or wheelbarrow) apparatus or a hand-held basket might be used for light-weight goods such as herbs and flowers.

Costermongers experienced a turbulent history, yet survived numerous attempts to eradicate their class from the streets. Programmes designed to curtail their activities occurred during the reigns of Elizabeth I, Charles I and reached a peak during Victorian times. However, the social cohesion within the coster community, along with sympathetic public support, enabled them to resist efforts to eradicate them.

They became known for their melodic sales patter, poems and chants, which they used to attract attention. Both the sound and appearance of costermongers contributed to a distinctive street life that characterised London and other large European cities, including Paris, especially in the 18th and 19th centuries. Their loud sing-song cry or chants used to attract attention became part of the fabric of street life in large cities in Britain and Europe. Costermongers exhibited a distinct identity. Individuals signalled membership of the coster community through a dress code, especially the large neckerchief, known as a kingsman, tied round their necks. Their hostility towards the police was legendary. The distinctive identity and culture of costermongers led to considerable appeal as subject-matter for artists, dramatists, comedians, writers and musicians. Parodies of the costermonger and their way of life were frequent features in Victorian music halls. Costermongers were ubiquitous in mid-Victorian England, but their numbers began to decline in the second half of the 20th century when they began to take up pitches in the regulated markets.

The term costermonger first appeared in written English in the early 16th century. The term coster is a corruption of costard, a kind of apple; and the word monger means a trader or broker. The first known use of the term costermonger occurs in the writings of Alexander Barclay, poet and clergyman, in the Fyfte Eglog of Alexandre Barclay of the cytezene and vpondyshman published around 1518. "I was acquaynted with many a hucster [=huckster], with a costardemonger and an hostler." The derivation of the term "costermonger" appears in Samuel Johnson's Dictionary of the English language, published in 1759. Charles Knight's London, published in 1851, also notes that a costermonger was originally an apple-seller. Although the original costermongers worked as itinerant apple-sellers, the word gradually came to refer to anyone who sold fresh fruit or vegetables from a basket, hand cart or temporary stall. The term can be used to describe anyone who sells goods outdoors or in the streets and has come to be a synonym for street vendor.^{[failed verification]}

Most contemporary dictionary definitions of costermonger refer to them as retail sellers or street vendors of fresh produce, operating from temporary stalls or baskets or barrows which are either taken on regular routes for door-to-door selling or which are set up in high traffic areas such as informal markets or lining the streets of busy thoroughfares. The Merriam-Webster Dictionary defines a costermonger as "a person who sells fruit and vegetables outside rather than in a store" while the Collins Dictionary defines a costermonger as "a person who sells fruit or vegetables from a cart or street stand.

Henry Mayhew, a Victorian social commentator, distinguished between itinerant and stationary costermongers in the following terms:

Under the term "costermonger" is here included only such "street-sellers" as deal in fish, fruit, and vegetables, purchasing their goods at the wholesale "green" and fish markets. Of these some carry on their business at the same stationary stall or "standing" in the street, while others go on "rounds." The itinerant costermongers, as contradistinguished from the stationary street-fishmongers and greengrocers, have in many instances regular rounds, which they go daily, and which extend from two to ten miles. The longest are those which embrace a suburban part; the shortest are through streets thickly peopled by the poor, where duly to "work" a single street consumes, in some instances, an hour. There are also "chance" rounds. Men "working" these carry their wares to any part in which they hope to find customers. The costermongers, moreover, diversify their labours by occasionally going on a country round, travelling on these excursions, in all directions, from thirty to ninety and even a hundred miles from the metropolis. Some, again, confine their callings chiefly to the neighbouring races and fairs.