A sump pump is a pump used to remove water that has accumulated in a water-collecting sump basin, commonly found in the basements of homes and other buildings, and in other locations where water must be removed, such as construction sites. The water may enter via the perimeter drains of a basement waterproofing system funneling into the basin, or because of rain or natural ground water seepage if the basement is below the water table level.

More generally, a "sump" is any local depression where water may accumulate. For example, many industrial cooling towers have a built-in sump where a pool of water is used to supply water spray nozzles higher in the tower. Sump pumps are used in industrial plants, construction sites, mines, power plants, military installations, transportation facilities, or anywhere that water can accumulate.

Sump pumps are used where basement flooding may otherwise happen, and to solve dampness where the water table is near or above the foundation of a structure. Sump pumps send water away from a location to any place where it is no longer problematic, such as a municipal storm drain, a dry well, or simply an open-air site downhill from the building (sometimes called "pumping to daylight").

Pumps may discharge to the sanitary sewer in older installations. Once considered acceptable, this practice may now violate the plumbing code or municipal bylaws, because it can overwhelm the municipal sewage treatment system. Municipalities urge building owners to disconnect and reroute sump pump discharge away from sanitary sewers. Fines may be imposed for noncompliance. Many homeowners have inherited their sump pump configurations and do not realize that their pump discharges into the sanitary sewer.

Sump pump systems are also utilized in industrial and commercial applications to control water table-related problems in surface soil. An artesian aquifer or periodic high water table situation can cause the ground to become unstable due to water saturation. As long as the pump functions, the surface soil will remain stable. These sumps are typically ten feet in depth or more; lined with corrugated metal pipe that contains perforations or drain holes throughout. They may include electronic control systems with visual and audible alarms and are usually covered to prevent debris and animals from falling in.

Sump pumps may be plugged into an electrical power receptacle. In this case, it is safer to use a dedicated circuit, which is less likely to lose power from a blown fuse or tripped circuit breaker. In addition, the dedicated circuit may not require GFCI protection, as it is less vulnerable to false tripping due to electrical noise, especially during thunderstorms. The dedicated circuit receptacle may be specially labeled to warn against unplugging the pump, or the plug may be attached using a special retaining bracket to discourage unplugging. Instead, the pump may be hardwired to electrical power, so that it cannot be unplugged.

Since a sump basin may overflow if not pumped, a backup system is important for cases when the main power is out for prolonged periods of time, as during a severe storm.

Some sump pumps can be automatically powered from a battery backup system, or a separate battery-powered system may be installed, typically with its float switch set slightly higher than the float switch of the primary pump.