Alexandra Polier, 27, a freelance journalist, spoke publicly in Nairobi, Kenya, to deny rumours of an affair. "I have never had a relationship with Senator Kerry, and the rumours in the press are completely false," she said.
Appealing to the media to respect her privacy, Ms Polier said: "Whoever is spreading these rumours and allegations does not know me ... I never interned or worked for John Kerry."
The denials will raise questions about press intrusion and accuracy, particularly in Britain, where the media was more willing to print Ms Polier's name and details of the alleged affair without any verification.
The allegations emerged on a rightwing website, the Drudge Report, on Thursday, two days after Mr Kerry opened up a commanding lead in the race to challenge George Bush for the presidency in November. The rumour was reported on the front page of the Times the following day.
Concerned about the veracity of the unsourced allegations, mainstream newspapers and broadcasting channels in the United States gave scant coverage to the story and did not publish Ms Polier's name. But it was picked up by newspapers abroad, particularly in Britain, where her name was used and her family tracked down and interviewed.
Mr Kerry, a senator from Massachusetts, is married to Teresa Heinz Kerry, heir to the $550m (£290m) ketchup empire. In an interview with Elle magazine last year, she said she had told her first husband she would maim him if he had an affair.
Asked last week whether there was truth to the rumour, Mr Kerry issued a blanket denial.
Ms Polier said she had waited before coming forward. "Because these stories were false, I assumed the media would ignore them. It seems that efforts to peddle these lies continue, so I feel compelled to address them."
While Matt Drudge, who runs the website, did not name Ms Polier, he said she had "recently fled the country, reportedly at Kerry's prompting".
A neighbour of the Poliers said Alexandra had wanted to go to Africa to visit the parents of her fiance.
In a separate statement, Ms Polier's parents, Terry and Donna Polier, of Malvern, Pennsylvania, dismissed the "completely false and unsubstantiated" allegations about their daughter.
Last week Mr Polier was quoted in the Sun as referring to Mr Kerry as a "sleazeball".
"He's not the sort of guy I would choose to be with my daughter," he was reported to have said.
Yesterday the parents said that Mr Kerry had not been involved with their daughter and they would be backing him for the highest office in the country.
"We love and support her 100% and these unfounded rumours are hurtful to our entire family," they said. "We appreciate the way Senator Kerry has handled the situation, and intend on voting for him."
Senator Kerry has won 14 of the first 16 primaries and caucuses to choose the Democrats' presidential nominee. He is widely expected to cement that lead with a convincing win in Wisconsin tonight.