Sharon Bialek, a former employee of a restaurant lobby group, said Mr Cain had reached up her skirt and pulled her head towards his crotch.
She is the fourth woman to accuse him of sexual misconduct in the last week. Are we seeing a pattern forming here?
The Cain campaign immediately issued a denial: “All allegations of harassment against Mr Cain are false.”
Mr Cain, a former pizza magnate who has never won an election, is a leading contender for the Republican nomination to take on President Barack Obama in 2012.
Washington news website Politico reported a week ago that at least two women had complained of sexual harassment while he was head of a lobby group, the National Restaurant Association, in the 1990s.
A third woman came forward days later, accusing Mr Cain of inappropriate behaviour at around the same time.
Ms Bialek is the first of the accusers to speak publicly.
She said she first met Mr Cain when she was seated next to him during a meal at the association’s annual convention. She described him as “warm and attentive” towards both her and her boyfriend at the dinner.
A month later, she was let go from her position as manager at the association’s education foundation.
Ms Bialek’s boyfriend suggested that she should contact Mr Cain to help her find other employment.
She said that Mr Cain had agreed to meet her in Washington DC in mid-July 1997, where he upgraded her hotel room.
Ms Bialek told reporters the groping incident occurred after they had dinner and Ms Bialek asked Mr Cain if he could help find her work at the state-level of the association.
“Instead of going into the offices he suddenly reached over and he put his hand on my leg, under my skirt toward my genitals,” she said. “He also pushed my head toward his crotch.”
Ms Bialek appeared to struggle to maintain her composure as she told journalists that she had asked Mr Cain what he was doing, to which he allegedly replied: “You want a job, don’t you?”
She was flanked at the Manhattan news conference by her lawyer, Gloria Allred, who has represented several high-profile clients, including an alleged mistress of golfer Tiger Woods.
Mr Cain acknowledged last week that one of his accusers received a financial settlement from the National Restaurant Association.
That woman – who reportedly won a $45,000 payout – issued a statement on Friday through her lawyer saying that Mr Cain had made a “series of inappropriate behaviours and unwanted advances”.
But despite being released from a confidentiality clause in the settlement, she said she wished to remain anonymous.