The American triumphed 7-6(3), 7-6(2) after two hours on the hard courts of Tokyo to claim his fourth career title.
Fritz's power hitting was on full display in the Japanese capital, putting Tiafoe on the back foot in the opening set, forcing him to save two break points just to reach the tiebreaker.
Tiafoe remained under pressure in the second set, but managed to save four break points, all while Fritz served two games to love. The 24-year-old then dominated the final tiebreak to earn a straight-set victory.
"[It is] crazy, I don't even think it's set in just how fast the last four or five days have been," said the Californian, as per atptour.com.
"It's so crazy, and I couldn't have written it any better. It's exactly what I needed for the Race [To Turin], for my ranking, to kind of put me in a good position for the end of the year, so it's amazing."
Speaking of rankings, Fritz's victory will see him break into the top ten in the world for the first time, reaching No.8 in the ATP rankings when they are made official on Monday.
It's been a breakthrough year for Fritz, who represented Team USA at the ATP Cup, won his first Masters 1000 title with a defining win over Rafael Nadal at Indian Wells and picked up his third title at the Eastbourne International in June.