Prevc, who won the men's large hill event at the Nordic World Ski Championships in Trondheim earlier this month, beat Austrian Stefan Kraft's previous mark of 253.5m from March 2017.
"I'm still shaking right now, really amazing," Prevc said after the jump in front of home fans in the Alpine valley in northwest Slovenia.
"I feel like I'm reading a book, or watching a movie and seeing this character do it, so I can't quite believe it, but I felt I could do anything after the take off and it was just perfect."
Despite Prevc's record-breaking jump, it was Slovenian compatriot Anze Lanisek who secured victory in Planica, marking his first Ski Flying World Cup win with a combined score of 482.1 points.
Prevc came second with 475.00 and Germany's Andreas Wellinger third with 455.8.