Marchand, the 'French Michael Phelps' who won the 400 Individual Medley on Sunday, won in an Olympic record time of one minute 51.21 seconds. Milak took the silver and Canada's Ilya Kharun the bronze.
Marchand will be chasing a third gold with the 200 breaststroke final later in the session.
Marchand's time of one minute 51.21 was the third fastest ever in the discipline.
The Frenchman, who had the Arena rocking when he won the 400 Individual Medley on Sunday and could yet end up with four golds, will be chasing his third title later in the 200 breaststroke final.
No Olympic swimmer - not even US great Michael Phelps who had the same coach as the Frenchman - has ever won the two events, but Marchand will start the breaststroke from lane four after qualifying with the fastest time.
He could make history even without winning the breaststroke as no swimmer has won medals in butterfly and breaststroke at the same Olympics.
