It was a fourth title for the enigmatic 22-year-old Australian, and his first since 2016.
Harrison had five breakpoint chances in the first set but couldn't convert, and third-seeded Kyrgios bounced back to get the decisive break in the seventh game and took control with a mix of unorthodox shot-making and pure power on his serve.
He closed with his 17th ace.
"It's a good feeling. As the week went on, I started serving better and hitting the ball better," Kyrgios said. "There was pressure on me today because I was the favorite and most people expected me to win, so I was proud of the way I handled that expectation and pressure."
Kyrgios had trouble with his left knee during the tournament, and needed to change the tape on the joint late in the first set of the final. After that, he appeared to move more freely and didn't expect it to trouble him at the Australian Open beginning Jan. 15.
His best run at an Australian Open was a quarterfinal exit in 2015. Kyrgios is hoping to improve on that after a confidence-boosting performance in Brisbane, where he beat defending champion and No. 3-ranked Grigor Dimitrov in the semifinals.
"For me, that's just confidence not only mentally but physically as well, and battling and fighting hard," he said. "It's good to see the work I was doing in the offseason is paying off. But pretty happy with my performance today."