Yuriko Koike's victory overshadows challenges from dozens of candidates vying to unseat the ruling party-backed incumbent.
Tokyo Governor Yuriko Koike has declared victory after exit polls projected she would win a third four-year term at the helm of Japan's influential capital.
With about 40 percent of the vote counted, Koike, 71, was leading by more than 1.29 million votes on Sunday, twice as many as his main rivals Shinji Ishimaru and Renho Saito, who had 664,000 and 603,000 votes respectively.
Official results are expected early Monday morning.
The vote was also seen as a test for Prime Minister Fumio Kishida's ruling party, which supports Koike, the first woman to lead the Tokyo city government.
Tokyo, a city of 13.5 million people with enormous political and cultural power and a budget equal to that of some nations, is one of the most influential political outposts in Japan.
A record 55 candidates challenged Koike, and one of the leading contenders was also a woman: liberal-leaning former lawmaker Saito, who usually uses only her first name, Renho, and was backed by opposition parties.
Minutes after exit polls projected her victory, Koike appeared at her campaign headquarters in Tokyo and celebrated by thanking the voters who elected her.
“I believe voters gave me a mandate based on my achievements over the past eight years,” said Koike, who pledged to push for more reforms and support Tokyo residents.
“I am fully aware of my great responsibility,” he said. “I will face my third term with all my heart and soul.”
Koike and her main rivals have notably pledged to improve the problems of low birth rates by expanding support for child-rearing, and Koike has promised to subsidize epidurals.
“After having their first child, I hear people say they don’t want to experience that pain again,” Koike said during the election campaign, according to local media.
“I want people to see childbirth and parenting as a joy, not a risk,” she said.
A Koike victory could help Kishida resist calls for his resignation from within the ruling Liberal Democratic Party (LDP) as his group's popularity wanes.
In April, the LDP lost three parliamentary by-elections to the opposition Constitutional Democratic Party of Japan (CDPJ), as well as the vote for the governorship of nearby Shizuoka Prefecture, which was won by Yasutomo Suzuki, a candidate backed by the opposition group.
Japan has also never had a female prime minister and a large majority of its lawmakers are men, even though Tokyo accounts for one-tenth of the nation's population and one-fifth of the economy.
