A sanitary sewer is an underground carriage system specifically for transporting sewage from houses and commercial buildings through pipes to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater. The system of sewers is called sewerage.

Sanitary sewers are distinguished from open sewers in that the sanitary system is closed off from its surroundings to limit interaction between the waste and the landscape that it travels through. They are also usually distinguished from combined sewers, which handle not only sewage but also surface runoff. Open sewers, consisting of gutters and urban streambeds, were common worldwide before the 20th century. In now-developed countries, large efforts were made during the late 19th and early 20th centuries to cover the formerly open sewers, converting them to closed systems with cast iron, steel, or concrete pipes, masonry, and concrete arches. Open sewers are not used in developed countries today, but they remain in use in some developing countries.

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