In marketing, market segmentation or customer segmentation is the process of dividing a consumer or business market into meaningful sub-groups of current or potential customers (or consumers) known as segments. Its purpose is to identify profitable and growing segments that a company can target with distinct marketing strategies.

In dividing or segmenting markets, researchers typically look for common characteristics such as shared needs, common interests, similar lifestyles, or even similar demographic profiles. The overall aim of segmentation is to identify high-yield segments – that is, those segments that are likely to be the most profitable or that have growth potential – so that these can be selected for special attention (i.e. become target markets). Many different ways to segment a market have been identified. Business-to-business (B2B) sellers might segment the market into different types of businesses or countries, while business-to-consumer (B2C) sellers might segment the market into demographic segments, such as lifestyle, behavior, or socioeconomic status.

Market segmentation assumes that different market segments require different marketing programs – that is, different offers, prices, promotions, distribution, or some combination of marketing variables. Market segmentation is not only designed to identify the most profitable segments but also to develop profiles of key segments to better understand their needs and purchase motivations. Insights from segmentation analysis are subsequently used to support marketing strategy development and planning.

In practice, marketers implement market segmentation using the S-T-P framework, which stands for Segmentation → Targeting → Positioning. That is, partitioning a market into one or more consumer categories, of which some are further selected for targeting, and products or services are positioned in a way that resonates with the selected target market or markets.

Market segmentation is the process of dividing mass markets into groups with similar needs and wants. The rationale for market segmentation is that in order to achieve competitive advantage and superior performance, firms should: "(1) identify segments of industry demand, (2) target specific segments of demand, and (3) develop specific 'marketing mixes' for each targeted market segment. " From an economic perspective, segmentation is built on the assumption that heterogeneity in demand allows for demand to be disaggregated into segments with distinct demand functions.

The business historian Richard S. Tedlow identifies four stages in the evolution of market segmentation:

The practice of market segmentation emerged well before marketers thought about it at a theoretical level. Archaeological evidence suggests that Bronze Age traders segmented trade routes according to geographical circuits. Other evidence suggests that the practice of modern market segmentation was developed incrementally from the 16th century onwards. Retailers, operating outside the major metropolitan cities, could not afford to serve one type of clientele exclusively, yet retailers needed to find ways to separate the wealthier clientele from the "riff-raff". One simple technique was to have a window opening out onto the street from which customers could be served. This allowed the sale of goods to the common people, without encouraging them to come inside. Another solution, that came into vogue starting in the late sixteenth century, was to invite favored customers into a back room of the store, where goods were permanently on display. Yet another technique that emerged around the same time was to hold a showcase of goods in the shopkeeper's private home for the benefit of wealthier clients. Samuel Pepys, for example, writing in 1660, describes being invited to the home of a retailer to view a wooden jack. The eighteenth-century English entrepreneurs, Josiah Wedgewood and Matthew Boulton, both staged expansive showcases of their wares in their private residences or in rented halls to which only the upper classes were invited while Wedgewood used a team of itinerant salesmen to sell wares to the masses.

Evidence of early marketing segmentation has also been noted elsewhere in Europe. A study of the German book trade found examples of both product differentiation and market segmentation in the 1820s. From the 1880s, German toy manufacturers were producing models of tin toys for specific geographic markets; London omnibuses and ambulances destined for the British market; French postal delivery vans for Continental Europe and American locomotives intended for sale in America. Such activities suggest that basic forms of market segmentation have been practiced since the 17th century and possibly earlier.