The world number 44 was only the fourth unseeded player to reach the Indian Wells final and the first since Kim Clijsters in 2005.
“Daria beat a lot of good players to get to the final," said Osaka after her 70 minute victory. “I knew that I would have to be very solid to have a chance of winning.
“It shows that by working hard and believing in yourself you can win titles.”
Osaka broke her Russian opponent to lead 5-3 and served out to pocket the opening set 6-3 in 39 minutes.
And prospects of a comeback looked shaky when Kasatkina lost her first service game in the second set. She was soon a double break down and trailing 4-1.
Osaka, who beat top seed Simona Halep 6-3 6-0 in the semi-final, displayed the same ruthlessness and broke Kasatkina for a fourth time to take the match.
"She was much better than me, so she really deserved to win," Kasatkina said. "Of course, we were both nervous at the beginning because it was the biggest final of our careers so far.
"But during the match, she was able to manage her nerves and I was still a little bit tight during the match. So it is what it is."
The ranking points from the title will push Osaka into the top 20. "At the start of the year, my goal was to play each match as well as I could and not to worry about rankings,” she said. "That will go up with wins."