A gutter is a pipe or trough along the edge of a roof that carries rainwater away from a building. Another kind of gutter is the indentation beside a street curb. Water flows through these gutters too, usually into a storm drain.

A rain gutter, also known simply as a gutter or guttering, is part of a building's water discharge system. It is a trough or channel that runs around the perimeter of a roof, and collects rainwater runoff from the roof, discharging it, usually to rainwater downpipes which convey it to a drainage system.

Gutters prevent water ingress into the fabric of the building by channelling the rainwater away from the exterior of the walls and their foundations. Water running down the walls causes dampness in the affected rooms and provides a favourable environment for growth of mould, and wet rot in timber.