The RSF-backed Prema driver finished over 3.5s clear of Alexander Albon who made it through from fifth at the start to take up the chase but was never able to get close enough to pose a significant threat.
Dennis's Prema team-mate and title contender Felix Rosenqvist wrapped up the final podium place a further six-tenths of a second in arrears after surging through from seventh on the grid.
Signature's Albon, along with Rosenqvist, benefited from the demise of Prema's other front-row starter Nick Cassidy who plummeted down the order to 25th on the opening lap.
The Japanese Formula 3 points-leader eventually finished two laps down in last place after clashing with Carlin's Callum Ilott..
Dennis had no such dramas to contend with opening up a 1.3s lead on the opening lap which he then extended at will - posting fastest lap along the way - as the race unfolded.
Championship leader Antonio Giovinazzi and title challenger Charles Leclerc both finished where they started in ninth and sixth places.
Rosenqvist's podium finish moves him to with 5.5 points of Giovinazzi at the top of the table while Dennis's win narrows the gap to Leclerc in third place to 39.5 points.
Update: Dennis who qualified fifth for race two will now start from fourth after Albon was stripped of pole for failing to provide the requisite fuel sample at the end of qualifying. The Signature driver will now start from the back of the grid.