Frier or Frierfjord is a fjord in Telemark county, Norway. The 14-kilometre (8.7 mi) long fjord is located along the Skaggerak coast in the traditional district of Grenland and it forms the border with the municipalities of Bamble, Skien, and Porsgrunn. The fjord is actually an inner arm of the Langesundsfjord, and historically (well into the 1700s) this fjord was also known as the Langesundsfjord. The fjord stretches from the opening to Langesundsfjord in the south to the mouth of the Porsgrunn River in the north. The much smaller fjord of Gunneklevfjord opens into the Porsgrunn/Skien River and is separated from Frierfjorden by the peninsula of Herøya.

The Frierfjorden narrows to a width of about 300 metres (980 ft) at Breviksstrømmen, the mouth of the fjord, where the town of Brevik sits on the northern side and the town of Stathelle lies on the southern side. The Brevik Bridge crosses Breviksstrømmen between the two towns. A little further into the fjord the newer Grenland Bridge crosses the fjord, carrying the European route E18 highway across Norway's highest cable stayed bridge.

The fjord has a great deal of commercial ship traffic, including to Rafnes and Herre in Bamble, to Norsk Hydro in Porsgrunn, and formerly to Norske Skog Union in Skien.

The fjord is named Frier (Old Norse: Friðir). The name is derived from the word fríðr which means "beautiful" or "lovely". The official name of the fjord is Frier. Frierfjorden is also an approved name, but that one is "not prioritized".

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  Brevik Bridge
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  Fjord system in Grenland, Telemark
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  Tug boats near Norsk Hydro at Herøya, Porsgrunn
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  Herøya, and the large industries

• List of Norwegian fjords