Williams saved 6 of 9 break points in her first meeting against the second-seeded Doi.
The seven-time major winner, a surprise first-round loser at the Australian Open last month, earned her 49th career WTA title.
"I've had so much success in Asia," Williams said after the match, having won her past two titles in China. "I feel like it's very lucky to play here."
The former No. 1 won the final three games to take the first set and lost just two more games to win the final in just under 90 minutes.
"The last five days were so exciting, just to feel the enthusiasm and to play great tennis every day," Williams said. "It was a perfect week for me, and the tournament did an amazing job with everything. The players and fans felt so welcome, so it was perfect."
Doi was bidding for her second WTA career title following last year's win in Luxembourg.
In doubles, hometown favorites Chan Yung-Jan and Chan Hao-Ching won their first title of 2016, defeating Japanese duo Eri Hozumi and Miyu Kato 6-4, 6-3.