Asada, first after the short program, began the free skate with a triple axel and was solid the rest of the way to finish with 204.62 points for her fourth straight national title.
"I'm glad that I had no major mistakes today," Asada said. "I was just trying to concentrate on my program and wasn't thinking about a fourth title but I did have the Olympics in mind."
Akiko Suzuki also booked a trip to Vancouver after finishing second with 195.90 points, 0.17 points ahead of Yukari Nakano.
Miki Ando, who had already secured a place in the Olympics, was fourth with 185.44 points.
The 19-year-old Asada had struggled in the International Skating Union's Grand Prix series this season and was looking to re-establish herself as a gold medal contender at the Olympics with a strong performance at nationals.
Asada will make her Olympic debut four years after she was left out of the Turin Games due to the age limit despite impressive results in senior events, including a victory at the 2005 Grand Prix finals.
"It's been a long four years," Asada said. "The Olympics have been a goal of mine for a long time and I'm very relieved."
Suzuki moved up from fourth place after the short program with a solid free skate. Her only mistake came when she lost her footing after a triple loop.
"I was able to skate up to my abilities today," Suzuki said. "It hasn't really sunk in yet that I'm going to Vancouver but I will do my best to prepare to show the world what I can do."
Daisuke Takahashi wrapped up a place in the Olympics on Saturday by winning the men's event and will join Nobunari Oda and Takahiko Kozuka in Vancouver. Oda secured his Olympic berth when he finished second in the Grand Prix finals earlier this month. Kozuka was third in the men's event Saturday.
Copyright 2009 The Associated Press.