She was expressionless as the verdict was read out in a hearing yesterday, local media reported. The defense filed an appeal.
According to the BBC, Kakehi had three other husbands who all passed away, though she has not been charged with any crime in relation to their deaths.
Before her arrest in November 2014 on a charge of killing her husband Isao, Kakehi denied murdering him, saying, "I would never do that because I would be suspected", as she was the only person living with the victim.
She also amassed around 1 billion yen ($8.8 million; €7.6 million) in inheritances over 10 years after she was made the beneficiary of her partners' life insurance policies.
Although she was aware of the toxicity of the cyanide compounds, she gave them to the four men by putting the substances in capsule form so that they looked like supplements, the judge said. He died on Dec 28, 2013, only a month after their marriage. The other men she is convicted of killing were former boyfriends aged 71 and 75. They later started a T-shirt printing business in Osaka. But following his death in around 1994, the factory went bankrupt and her house was put up for auction, leading her to ask neighbors for a loan. His death had not been seen as suspicious but was the first of many eventually linked to her.
The prosecution's case rested on a small bag of cyanide found in a plant pot that Kakehi had tried to discard.
Defense lawyers argued Kakehi shouldn't be liable for the crimes because of a dementia diagnosis.
They added that legal autopsies have not been performed on some of the victims, and Kakehi, who has early onset dementia, was not a reliable witness in the 135-day trial. I have no intention of hiding the guilt.
"Even if I were executed tomorrow, I would die smiling", Kakehi previously told the judges. But she quickly retracted her confession.
While sentencing the old woman, presiding judge Akiko Nakagawa said: "It was an extremely malicious and sneaky crime borne out of greed for money".