Osaka, supremely confident and in top form coming into the final off the back of 10 consecutive wins, looked anxious throughout the contest and allowed Pliskova to win easily in just over an hour.
The Czech former world number one came into the contest having spent 419 minutes on court this week, more than any other player despite a first round bye and almost twice as long as Osaka.
It was the Japanese player who appeared the more fatigued, however, and Osaka, who had not dropped a set during the tournament, hit a series of unforced errors and showed none of the poise and power that secured her a first Grand Slam title.
Pliskova was more than happy to take advantage, capitalising both times when presented with a break point and winning the last 11 points to secure her second WTA title of the campaign.
Osaka has now lost in the final of this tournament twice, having also been runner-up in 2016.
In clinching the title, Pliskova has taken a huge step towards securing her berth at the season WTA Finals.
Pliskova is ninth in the race, with the top eight players qualifying for Singapore. Kiki Bertens, currently in eighth place, competes in the final of the Korea Open later on Sunday.