Fjord or Fiord (Old Norse Fjörthr), inlet of the sea, or a narrow bay, extending far inland between steep walls of rock. Fjords usually have branches that jut from the main body of water. The walls of a fjord extend deep below the surface of the water. Fjords tend to be most shallow near the mouth, and deepest inland.

Most geologists believe that fjords were formed when glaciers carved deep grooves in the coastline. Later these grooves filled with water as the sea level rose. Many fjords began as river mouths and were deepened by glaciers.

Many of the world's fjords are located along the coast of Norway. Sognafjorden, Norway's longest fjord, extends inland for 204 km (127 mi) and attains a depth of 1,308 m (4,291 ft). Fjords are also found along the coasts of British Columbia, Alaska, Iceland, Greenland, Nova Scotia, Maine, southern Argentina, and New Zealand. Inlets called sea lochs, or firths, which mark the coast of Britain, are similar to fjords.