Submergent coastlines or drowned coastlines are stretches along the coast that have been inundated by the sea by a relative rise in sea levels from either isostacy or eustacy.

Submergent coastline are the opposite of emergent coastlines, which have experienced a relative fall in sea levels.

Many submergent coastlines were formed by the end of the Last Glacial Period (LGP), when glacial retreat caused both global sea level rise and also localised changes to land height.

Submergent coastlines form either when sea level rises or the land level fall. This can be caused by isostatic or eustatic change. Both isostatic and eustatic change can be caused by a variety of reasons.

Isostatic change could be due to post-glacial adjustment. Another common cause of isostatic change that can result in a submergent coastline is tectonic action. Events such as earthquakes or volcanic eruptions may cause land subsidence therefore causing isostatic sea level increases.

Eustatic sea level rise can also be caused by many reasons. One common reason is thermal expansion. Thermal expansion occurs when water gets warmer and so the volume of water increases, this therefore causes global sea level rise forming submergent coastlines.

Features of a submergent coastline include:

Rias, a drowned river valley. They are a section of a river valley flooded by the sea, making them much wider than would be expected based on the river flowing into it. Many rias were formed by the rise in sea level after the melting of vast continental glaciers. Rias commonly have a widening funnel shape and gradually increasing depth as they move towards the coast. The widening and deepening of the ria towards the sea usually causes an exaggerated tidal effect within the estuary.