The Pole broke Pegula five times and won the final 11 games in a 59-minute slaughter to capture her first WTA Finals title in a match delayed by a day due to rain.
“I want to thank my team, who have been with me for the whole season,” Świątek said during the trophy ceremony.
She also thanked the fans who endured countless rain delays and gusting winds over the course of the event at the coastal Mexican resort town.
“Thank you for all these Polish flags and Polish people who are coming to every tournament, and those from many countries. Every year, I feel like I have more and more support.”
Świątek came into the tournament red-hot after winning last month's China Open and never dropped a set over the course of her five matches at the event, which brings together the top eight players in the world.
The Polish player ends the season on an 11-match winning streak and will finish the year in the top spot for the second year in a row, having lost the position to Sabalenka after the U.S. Open.
Świątek, 22, collected six out of her total of 17 career titles this season. She retained the French Open crown in June to lift her fourth Grand Slam trophy.
The 29-year-old Pegula, who struggled on her serve and failed to match Świątek's energy level, was gracious in defeat.
“Congrats on ending the year number one and winning this tournament; it's really incredible,” she said.
“You continue to push me to be a better player, and I appreciate that aspect of it.”
