A ria is a submergent coastal landform, often known as a drowned valley or drowned river valley. Rias are almost always estuaries. Rias form when sea levels rise or plate tectonics cause coastal levels to fall. When this happens valleys which were previously at sea level become submerged. The result is often a very large estuary at the mouth of a relatively insignificant river.

The south coast of England is a submergent coastline, and contains many rias, including Portsmouth harbour, Southampton water, Poole harbour; the estuaries of the Exe, Teign and Dart, the Kingsbridge estuary, and Plymouth Sound in Devon; and the estuaries of the River Fowey and River Fal in Cornwall. Chesapeake Bay and Delaware Bay, on the east coast of the United States, and San Francisco Bay, on the Pacific coast, are rias.

The Kingsbridge estuary is an extreme example of a ria forming an estuary disproportionate to the size of its river, because no river flows into it at all, only a number of small streams.