Summer Zervos, who alleges Mr Trump forced himself on her in 2007, said he lied to the nation about his behaviour.
The lawsuit, which Ms Zervos announced at a news conference, alleges Mr Trump is a "liar and misogynist" who "debased and denigrated" her.
The news came just three days before Mr Trump is sworn in as president.
"Since Mr Trump has not issued a retraction, as I requested, he has therefore left me with no alternative but to sue him in order to vindicate my reputation," Ms Zervos said at a news conference.
