A fjord (pronounced FEE-ord or fyord, SAMPA: ['fi:3:d] or ['faI3:d]) is a glacially overdeepened valley, usually narrow and steep-sided, extending below sea level and filled with salt water.

Fjords are found in locations where current or past glaciation extended to sea level. A fjord is formed when a glacier (carving its typical U-shaped valley) meets the sea and melts. This leaves a narrow, steep sided valley into which the sea floods. The flood creates a narrow, deep lake (sometimes as deep as 1300m) connected to the sea. The terminal moraine pushed down the valley by the glacier is left underwater at the fjord's entrance, causing the water at the neck of the fjord to be shallower than the main body of the fjord behind it.

This shallow threshhold and the protection afforded by the valley's sides generally means that fjords are excellent natural harbours. Consequently fjords often provide the home-port to fishing fleets, and in industrialised locations have come to be used for fish farming and ship building.

The word fjord comes from Norwegian, meaning "narrow steep-sided valley extending below sea level and occupied by salt water."

Fjords are found on the west coasts of Norway and New Zealand, and on the south and west coasts of Alaska. The west coast of Scotland also features fjords (called "sea lochs"), and the long fjord-like bays of the New England coast are sometimes referred to as "fiards". The largest fjord in the world is Sognefjorden in Norway.