Osaka ousted Maria Sharapova, fifth seed Karolina Pliskova and then world number one Simona Halep in the semi-finals on her way to the championship.
She won the final three games of the first set, then dominated the second, needing only 70 minutes for the win.
"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."
After several unforced errors to open the first set, the youngster, who resides in the United States, took control of the tie.
She won five consecutive games from 3-3 in the first before Kasatkina got on the scoreboard in the second.
Up 5-2, Osaka closed out the match with a backhand winner.
"She probably wasn't going to give me any balls so I just had to stay very consistent," Osaka said.
Using her explosive power, Osaka delivered 23 winners to Kasatkina's 10 and won 79 percent on her first serve.
Meanwhile the Russian, perhaps fatigued, could not produce the sterling play that led to her giant-killing win over American Venus Williams in the semi-finals.
The victory will enable Osaka to rise to a career-best 22nd when the new world rankings are announced on Monday.
"In the beginning of the year, I just wanted to play every match really well," she said. "Just focus one point at a time.
"I think I am doing that so I don't really mind where my ranking is because I am sure at the end of the year if I keep playing well like this it will be pretty high."
But she will need to practice on her acceptance speeches.
"Hi, I'm Naomi,'" she started Sunday's, quickly adding "OK, never mind."
Known for her shyness, she was definitely more at home on the court than accepting her first trophy.
"This is probably going to be like the worst acceptance speech of all-time," she said after forgetting sponsors and supporters she wanted to thank.
(Reporting by Gene Cherry in Salvo, North Carolina; Editing by Christian Radnedge)
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