Chisako Kakehi, aged 70, said she used a cyanide compound to kill her fourth and most recent husband in December 2013. She is also accused of murdering two previous husbands and attempting to murder a boyfriend.
"I have no intention of hiding the guilt," Ms Kakehi told the court in the city of Kyoto, after admitting the murder of 75-year-old Isao Kakehi.
"I will laugh it off and die if I am sentenced to death tomorrow."
Reports in Japan say as many as ten men that she was either in a relationship with, or associated with, are known to have died.