It was the eighth Olympic medal for Bjoergen, four of which have been gold.
Bjoergen and Charlotte Kalla of Sweden pulled away from the pack over the final 200 meters and they then sprinted for the line at the end of a 15-kilometer race that took more than 38 minutes to ski.
The sprint was won by Bjoergen, the defending champion in this race who finished 1.8 seconds in front of her Swedish rival in a time of 38:33.6.
Heidi Weng of Norway wound up third, 18 seconds behind, outdueling Therese Johaug of Norway and Aino-Kaisa Saarinen for the bronze. The first five finishers all collapsed at the finish and remained sprawled across the snow while other competitors worked their way around them.
Bjoergen won five medals in Vancouver four years ago, more than any other athlete. In addition to the skiathlon, she won the sprint title and was a member of the winning cross-country relay team.
The skiathlon race consists of two 7.5-kilometer legs -- one of them contested in the classic form of cross-country skiing and the other in freestyle. Each skier makes a pit stop between the two legs to change skis.
Bjoergen had a narrow lead after the classic portion of the race and although her pit stop time of 35.1 seconds was not fast, she quickly made up the lost time.
The top American finisher was Jessica Diggins, who crossed the line in eighth place. She wound up a minute and a half behind Bjoergen