Aprilia's Marco Bezzecchi came third for his first sprint podium since 2023 while pole sitter Fabio Quartararo of Yamaha crashed with four laps to go.
Marc extends his lead in the championship to 43 points over Alex going into Sunday's race while Ducati's Francesco Bagnaia, third in the championship, could only manage fifth place behind VR46 Racing's Fabio Di Giannantonio.
The victory will give Marc a huge boost of confidence after he crashed twice in Friday's practice, as he seeks a first race win at Assen since 2018.