Vefsnfjorden[1] is a fjord in the traditional district of Helgeland in Nordland county, Norway.

It is about 51 kilometres (32 mi) long, reaching a maximum depth of about 440 metres (1,440 ft) below sea level.

The fjord heads northeast to the village of Sundøya at the northern end of Alsta where a small channel connects to the Leirfjorden.

At this point, the fjord turns to the southeast as it proceeds inland to the town of Mosjøen where it ends.

A German prisoner ship was sunk in the Vefsnfjorden by British Aircraft during World War II with major loss of life.