Second seed Swiatek dominated Pegula from start to finish to clinch a 6-1, 6-0 victory in a 59-minute masterclass.
The 17th win of Swiatek's career ensures that the four-time major singles champion from Warsaw will finish the season as world number one for a second straight year.
"The team that's been with me the whole season, we had many ups and downs," a jubilant Swiatek said after her win.
"This is for sure an up. We will for sure have many more if we keep working like that," added Swiatek, who lost just 20 games across the tournament -- a new record.
Pegula had few complaints about her defeat after an error-strewn display.
"It just was one of those days where I felt like I was going for too much," Pegula said. "And she was just playing super solid."
Swiatek, 22, had beaten outgoing world number one Aryna Sabalenka in the semi-finals on Sunday. Sabalenka took over the number one ranking from Swiatek following the US Open in September.
Pegula defeated Swiatek in the two players' previous meeting in Montreal in August, and had impressed on her way to Monday's final, which had been shunted back 24 hours after wet weather washed out play on Saturday.
But the 29-year-old American from New York never looked capable of pulling off a repeat of her Canadian victory against three-time French Open champion Swiatek, who was sharper in every facet of her game.
The Polish star dominated on serve, and pulled Pegula all over the court with whipped groundstrokes that left her opponent flailing.
Pegula also struggled on serve, being broken five times while coughing up 23 unforced errors compared to Swiatek's six.
Swiatek -- who gave up only one break point opportunity in the entire match, which Pegula was unable to convert -- sealed victory in the sixth game of the second set when the American pulled a backhand return long.
Swiatek took control of Monday's contest early on, breaking Pegula in the fourth game for 3-1 before holding and breaking again for a 5-1 lead.
She served out to love to claim the first set and then carried that momentum into the second set.
She broke Pegula twice in the first three games of the second set, and then held comfortably for a 4-0 lead.
Pegula -- who won just seven points on first serve through the match -- suffered another break to leave Swiatek on the brink.
Loud cheers erupted for Pegula when she finally managed to earn her first break point of the match on Swiatek's serve.
But Swiatek quickly got it back to deuce and then moved to match point by crushing a forehand winner with Pegula stranded, before wrapping up victory on the next point after another errant groundstroke from the American.
