Joy Villa says Mr Lewandowski hit her twice on the backside during an event in Washington last month.
She told the US media she spoke to the police on Christmas Eve, having been persuaded to launch a formal complaint by friends.
Mr Lewandowski has not responded to US media's requests for comment.
The accusations are the latest in a long line of sexual harassment and assault claims linked to celebrities, politicians and companies, which have gathered pace in the past year, particularly following the downfall of film mogul Harvey Weinstein.
