Continuing the form that took her to the US Open semifinals, Wozniacki, who lifted the title in the Japanese capital six years ago, returned to the final after a thrilling 4-6, 7-5, 6-4 victory over the No.2 seed.
Radwanska had her chances, serving for the match at 5-3 in the second set, but ultimately Wozniacki, who committed just 29 unforced errors during their two hours and 51 minutes on court, was just too consistent.
"I think every time I play her, we play really tough matches - and long ones. Luckily I managed to win that second set and then we both fought so hard in that third one," Wozniacki said.
"I don't really think too much about it," Osaka said. "I just know [Wozniacki] is gonna get a lot of balls back and I feel like I'm just going to have to be patient and stick to the game plan - when I think of a plan."
Osaka entered the tournament having never won a match at Toray, and has now become the first Japanese finalist since Kimiko Date-Krum 21 years ago. It's also the first WTA-level final for the 18-year-old.
"I don't really feel pressure here since the first match, since everyone is ranked higher than me. I'm a wildcard, so I just try to do my best. But this, for sure, it's my best tournament."