Scott will continue Team GB's dominance in the event, winning the country's fifth successive Olympic title.
The 29-year-old finished eighth and second in his two races on Sunday to lead by 24 points.
A sailor's worst finish is not included in their final score, so the most Scott can score in the medal race is 17.
Sir Ben Ainslie, who won gold in London and Beijing, congratulated his successor for winning "in serious style".
Britain's Giles Scott is guaranteed Olympic gold in the men's Finn class after opening an unassailable lead before Tuesday's medal race.