The 20-year-old arrived at the tournament in blistering form and went into Sunday's final as favourite following her impressive run of results.
But few expected Osaka to make such short work of Kasatkina, with the Japanese star racing to a 6-3 6-2 win.
Australian Open
'I felt a little rattled' - Barty reveals frustrations over world No 1 questions
23/11/2021 AT 11:31
A crunching backhand volley on match point earned Osaka her first title and crown of "the queen of the desert" in California.
The overjoyed youngster raced to embrace her family after shaking hands with the umpire, and must now be considered one to watch for the remaining three Grand Slams of the season.
"If you put in a lot of hard work you can play well," said Osaka, who was playing in only her second final after being the runner-up in Tokyo in 2016. "If you try and believe in yourself, you can win."
"She probably wasn't going to give me any balls so I just had to stay very consistent," she added.
Tennis
Osaka 'in shock' at Peng Shuai disappearance after sexual allegations
16/11/2021 AT 22:27
WTA Finals
Six first-timers, but whose run to WTA Finals is most surprising?
10/11/2021 AT 06:42