In geography, coastal erosion is a process which affects the landmass of an area as a consequence of the sea acting upon it. Most often the primary causative factor is tidal erosion of the landmass. The direction of tidal or other current flows can also be of significance: for example, with the sea running parallel to or at an angle to the coastline, sand and shingle is quickly stripped away, sometimes to devastating effect, as was the case at Dunwich, the capital of the English medieval wool trade, which disappeared over the space of a few centuries.

This process can be dramatically accelerated by storms. Human interference can also adversely affect natural coastal defences: Hallsands in Devon, England, was a coastal village that was washed away overnight as the consequence of dredging for shingle in the bay in front of it.

Coastal erosion can create dramatic rock formations in areas where the coastline contains stones of varying densities. The softer areas become eroded much faster than the harder ones, creating tunnels, bridges, columns, pillars and other anomalies.

See also: natural arch, blowhole