"Well, I'm a serial killer," she reportedly announced.
The woman, whose identity has not been released, proceeded to slash and stab her date several times as he tried to escape, according to news station KOMO.
The injured 29-year-old, who insists he met the woman on a website for dating — not just sex — was eventually able to flee and call police from the motel office.
He was taken to Harborview Medical Center with serious injuries including a punctured lung. However, he is expected to recover.
His attacker was found by police in the parking lot, with the knife still in her pocket.
When they confronted her, they said she confessed she had planned to eat her date's heart.
She had even written a note that she planned to leave on his body, saying would kill again, officers said.
"I'm a loon", she allegedly told interviewing officers.
The suspect has not yet been charged, but is being held on US$1 million bail.