In economics, the tertiary sector (also known as the service sector) is the economic sector which comprises the provision of services as opposed to the manufacture of finished goods. Services (also known as "intangible goods") include attention, advice, access, experience and affective labour.

The tertiary sector involves the provision of services to other businesses as well as to final consumers. Services may involve the transport, distribution and sale of goods from a producer to a consumer, as may happen in wholesaling and retailing, pest control or financial services. The goods may be transformed in the process of providing the service, as happens in the restaurant industry. However, the focus is on people by interacting with them and serving the customers rather than transforming the physical goods. The production of information has been long regarded as a service, but some economists now attribute it to a fourth sector, called the quaternary sector.

It is sometimes hard to determine whether a given company is part of the secondary or the tertiary sector. It is not only for-profit companies that make up the sector in some schemes, but also governments and government agencies (such as the police or military) and nonprofit organizations (such as charities or research associations) that provide services.

To classify a business as a service, one can use a classification system such as the United Nations' International Standard Industrial Classification, the North American Industrial Classification System (NAICS), the Statistical Classification of Economic Activities in the European Community (NACE) or similar systems elsewhere. These governmental classification systems have a first level of hierarchy that reflects whether the economic goods are tangible or intangible.

For purposes of finance and market research, market-based classification systems such as the Global Industry Classification Standard and the Industry Classification Benchmark are used to classify businesses that participate in the service sector. Unlike governmental classification systems, the first level of market-based classification systems divides the economy into functionally related markets or industries. The second or third level of these hierarchies then reflects whether goods or services are produced.

For the last 100 years, there has been a substantial shift from the primary and secondary sectors to the tertiary sector in industrialized countries. This shift is called tertiarisation. The tertiary sector is now the largest sector of the economy in the Western world, and is also the fastest-growing sector. In examining the growth of the service sector in the early nineties, the globalist Kenichi Ohmae noted that:

In the United States, 70 per cent of the workforce works in the service sector; in Japan, 60 per cent, and in Taiwan, 50 per cent. These are not necessarily busboys and live-in maids. Numerous of them are in the skilled category. They are earning as much as manufacturing employees, and often more.

Economies tend to follow a developmental progression that takes them from heavy reliance on agriculture and mining, toward the development of manufacturing (e.g. automobiles, textiles, shipbuilding, steel) and finally toward a more service-based structure. The first economy to follow this path in the modern world was the United Kingdom. The speed at which other economies have made the transition to service-based (or "post-industrial") economies has increased over time.