The two-time Australian Open dropped only nine points on serve and hit 10 aces as she overcame some jitters late in the second set in an otherwise confident performance to pick up her sixth WTA 1000 title.
Pegula had appeared to be in good form after she retained her title in Toronto earlier this month but she struggled with her serve, with five double faults.
Sabalenka got the break in the fourth game after Pegula whacked a forehand into the net and the Belarusian built on the momentum with a hold to love in the fifth, closing out the first set with an unreturnable serve.
She sent over a barrage of powerful forehands to get another break in the opening game of the second set but helped Pegula to get her lone break point with a double fault and a pair of unforced errors in the 10th game.
Sabalenka broke back immediately, however, and blew kisses to the crowd after she forced her opponent into an error on match point. She will move up one spot to number two in the rankings ahead of the US Open, which starts next week.
The performance removed any possible doubt that Sabalenka is one of the hot favourites in Flushing, despite a shoulder injury earlier this season that forced her to miss Wimbledon.
She cruised through Cincinnati without dropping a set -- including against the world number one Iga Swiatek -- and will be out for revenge in New York after finishing runner-up last year.
"I want to thank my team -- we've been through a lot but we never stopped working, never stopped improving ourselves and I'm really happy to have you by my side," Sabalenka said in on-court remarks.
Pegula had mistakenly used the name "Serena" instead of "Aryna" when talking about the Belarusian in an on-court interview after her semi-final, and she joked on Monday that the mix-up with the 23-times Grand Slam winner was fitting.
"It felt like Serena (Williams) today with the way you were serving there," she said at the trophy ceremony. "I may have wanted Serena instead of Aryna."
