The Czech needed to save one in the next game but then came her big chance, three more break points coming at 0-40 versus the Osaka serve.
Her second consecutive grand slam title ensures she will be the new world No 1 and on this evidence, she might stay there for some time.
The 21-year-old Japanese star also becomes the first woman since Jennifer Capriati in 2001 win the following major after her maiden grand slam triumph.
The hard-fought victory made Osaka the first Asian, male or female, to hold the world's top ranking, taking over from Romania's Simona Halep.
Osaka had two set points against serve at 6-5, thwarted by a forehand victor and service victor.
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On the women's side, all eyes will be on Serena Williams as she aims for Grand Slam number 24.
Osaka soon steadied early in the decider, nabbing the first break behind a big backhand victor and took another 0-40 lead at 4-2.
Kvitova has yet to be challenged in the Australian Open, winning all matches in straight sets.
Osaka has played only ranked opponents thus far. She broke back and then went ahead for 3-2 when the wheels came off for the Czech, spraying errors in being broken to love. She headed to the locker room after the second set with a towel draped over her head.
Kvitova would not surrender, saving three break points before again forcing Osaka to serve it out.