The Sognefjord or Sognefjorden (Urban East Norwegian: [ˈsɔ̂ŋnəˌfjuːɳ], English: Sogn Fjord), nicknamed the King of the Fjords (Norwegian: Fjordenes konge), is the longest and deepest fjord in Norway. Located in Vestland county in Western Norway, it stretches 205 kilometres (127 mi) inland from the ocean to the small village of Skjolden in the municipality of Luster.

The fjord gives its name to the surrounding district of Sogn. The name is related to Norwegian word súg- "to suck", presumably from the surge or suction of the tidal currents at the mouth of the fjord.

The fjord runs through many municipalities: Solund, Gulen, Hyllestad, Høyanger, Vik, Sogndal, Lærdal, Aurland, Årdal, and Luster. The fjord reaches a maximum depth of 1,308 metres (4,291 ft) below sea level, and the greatest depths are found in the central parts of the fjord near Høyanger. Sognefjord is more than 1,000 metres (3,300 ft) deep for about 100 kilometres (60 mi) of its length, from Rutledal to Hermansverk. Near its mouth, the bottom rises abruptly to a sill about 100 metres (330 ft) below sea level. The seabed in Sognefjord is covered by some 200-metre-thick (660 ft) sediments such that the bedrock is some 1,500 metres (4,900 ft) below sea level. The fjord is up to six kilometres (3+1⁄2 mi) wide. The average width of the main branch of the Sognefjord is less than five kilometres (3 mi). The depth increases gradually from Årdal to a central basin reaching more than 1,000 metres (3,300 ft) in depth located between Leikanger and Brekke. From Brekke the floor rises rapidly to Losna island, then drops gradually with a threshold at about 150 metres (500 ft) in the Solund area. Thresholds occur in an area with sounds, valleys, and low land where the glacier was allowed spread out and lose its erosive effect.

Cliffs surrounding the fjord rise almost sheer from the water to heights of 1,000 metres (3,300 ft) and more. Around the outer area the land rises to about 500 metres (1,600 ft) above the sea, while in the inner areas of the fjord, they reach about 1,600 metres (5,200 ft). The inner part has extensive tributary fjords such as Aurlandsfjorden, while the outer part is connected by narrow sounds to neighbouring fjords. Near the coast the fjord mouth is bounded largely by low islands and skerries that are part of the strandflat.

The inner end of the Sognefjord is southeast of a mountain range rising to about 2,000 metres (6,600 ft) above sea level and covered by the Jostedalsbreen, continental Europe's largest glacier. Thus the climate of the inner end of Sognefjorden and its branches are not as wet as on the outer coastline. Hurrungane range at the eastern end of the fjord reaches 2,400 metres (7,900 ft). The greatest elevation from seabed to summit is at Sogndal. Several rivers pour fresh water into the fjord with an annual "spring" flood in June. The mouth of the fjord is surrounded by many islands including Sula, Losna, and Hiserøyna. The Sognefjord cuts through a northwestern gneiss area with a south-west to north-east structure, and penetrates the Caledonian fold through in the inner part. There is no clear relation between the east–west direction of the main fjord and the fold patterns of the bedrock, while some of tributary fjords in the parts corresponds to fold pattern.

The volume of the whole Sognefjorden including its various branches is about 500 cubic kilometres (120 cu mi), while the total volume of rock eroded by glaciers from the entire Sognefjord system and adjacent valleys is about 4,000 cubic kilometres (960 cu mi).

There are many smaller fjords which branch off the main fjord.

The innermost arm of the Sognefjorden is called the Lustrafjord, in the municipality of Luster. At its end is the village of Skjolden, which is an access point to Jotunheimen National Park. In earlier times, transport between Bergen and the Scandinavian inland was by boat between Bergen and Skjolden and from there on a simple road over the highlands (today Norwegian County Road 55), or by boat to Lærdal and through the mountain pass to Valdres (now European route E16).