Chisako Kakehi was sentenced to death in 2017 for killing three men, including her own husband, with cyanide, and for the attempted murder of a fourth man. The sentence was later upheld by a higher court.
She was confirmed dead at a hospital on Thursday where she was taken after being found lifeless in her cell. The Japanese Ministry of Justice announced on Friday.
Kakehi was 78 years old. The cause of death is not known.
Kakehi had a series of relationships with older or sick men and over the course of ten years secured payouts of tens of millions of kroner from the men’s life insurance policies. She met the men via dating services where she specified that she was looking for wealthy, childless partners.