The Pole, a double Olympic champion and a two-time world champion, was the fastest woman in the classic-technique race, which attracted almost 10,000 competitors on Saturday.
Kowalczyk finished the marathon in 3 hours, 6 minutes, 10.7 seconds, beating second-placed Kateřina Smutná of the Czech Republic by more than 3 minutes. Astrid Øyre Slind of Norway was the third-fastest woman, with a time of 3:10:05.4.
Meanwhile, Andreas Nygaard of Norway was the fastest man and won the marathon overall, clocking in at 2:33:13.6.
The marathon took place from Rena, a small town in eastern Norway, to the ski resort of Lillehammer and marked the second time Kowalczyk triumphed after she won the Birkebeinerrennet last year.
The race has been held annually since 1932, and commemorates a journey made by knights to save the infant pretender to the Norwegian throne, Håkon Håkonsson, in 1206.
All competitors must carry a backpack weighing 3.5 kg, symbolising the weight of the then-infant king.
At a press conference a day before the race, Kowalczyk said she would focus on competing in cross-country ski marathons in the future at the expense of World Cup appearances, according to a report by the biegowki24.pl website.
At the start of the season, the 35-year-old Pole, who has four overall World Cup titles under her belt, said that vying for the World Cup’s Crystal Globe would not be a priority for her this year.
Meanwhile, Heidi Weng of Norway claimed this season’s Crystal Globe as the 2018 FIS Cross-Country World Cup Finals ended in Falun, Sweden, on Sunday.
Kowalczyk has five Olympic medals and eight world championship medals to her name, but has been struggling to return to top form ever since she was diagnosed with depression in 2014.
Source: TVP, sport.pl, biegowki24.pl