Kanae Kijima, 37, known as Black Widow, murdered three men aged 41, 53 and 80 for financial gain, the court said. The judge added that there was no room for leniency.
She met the men on internet dating sites and poisoned them with carbon monoxide by burning charcoal briquettes after giving them sleeping pills.
Kanae Kijima says she is innocent and plans on appealing against the verdict.
The case has gained notoriety in Japan as the “Black Widow” case, named after the female spider that eats its partner after mating.
More than 1,000 people queued up for fewer than 50 seats at the court to hear the verdict.
“The defendant repeatedly committed quite serious crimes that claimed the lives of three people,” the presiding judge, Kazuyuki Okuma, in Saitama district court, was quoted by the Kyodo news agency as saying.
“There is no room for leniency as the defendant committed the crimes for selfish purposes. She reiterated irrational excuses in court and did not show any remorse.”
Prosecutors said she killed the men so she would not have to pay back the money they had given her.
The defense said the men had either committed suicide because Kanae Kijima broke up with them, or died accidentally.
Two of the three victims were found dead in their homes in Tokyo’s Chiba prefecture. The third was found in a rented car in Saitama prefecture.
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