The 26-year-old was an impressive 42 seconds ahead of the second placed Marit Bjørgen, also of Norway.
Just 1.4 kilometres into the race, Therese Johaug was over 10 seconds ahead and it began an easy win for the Norwegian.
Justyna Kowalczyk of Poland, Charlotte Kalla of Sweden, and Marit Bjørgen were the closest during the run, but none of them came close to catching Johaug's incredible run.
Bjørgen was the closest of the challengers and secured a Norwegian double.
"Therese did a terrific run today. There was no way to do anything about her. I lost a lot of time during the start, so I understood that I had to focus on getting second place,” said Bjørgen to NTB.
Therese Johaug. Photo: Terje Bendiksby / NTB scanpix
There was more good news for Johaug when she announced a new boyfriend, fellow sports star Nils Jacob Hoff, reported TV 2.
Hoff, 29, is a world champion rower, originally from Bergen.
At first, Johaug was hesitant to confirm the romance rumours connected to her and Hoff, but she did tell the media: "Yes, we are partners, but I don't want to comment more than that.”