Backwater is a term used to describe the unpotable water contained in sanitary (sewage) or rainwater drainage systems when flow blockages in these lines cause the contents to spill out in areas where it is not desired.

Each year in the United States, sewers back up in basements an estimated 400,000 times and municipal sanitary sewers overflow on 40,000 occasions, dumping potentially deadly pathogens into the nation's streets, waterways and beach areas. Many homeowners have experienced the unfortunate results when raw, contaminated sewage comes up through basement drains and plumbing fixtures causing significant damage to their home. Backwater valves are specifically designed into sanitary and rainwater drainage systems to protect against such backups.

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Backwater is also a term used to describe water held back by a dam or levee. The stagnant or sluggish nature of this water also gives rise to a further metaphorical use of the term, to describe anywhere seen as being stagnant or boring.

The Kerala Backwaters are a series of lagoons and lakes in Kerala, India.

Backwater is the name of a song by Brian Eno, from his album Before and After Science.