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 told journalists in Los Angeles.
She was joined by her lawyer, Gloria Allred, a Democratic party activist, who said the plaintiff's allegations had passed a lie detector test.
Ms Zervos is one of several women who came forward to accuse Mr Trump of s3xual assault in the weeks leading up to the presidential election.
The 41-year-old former reality show contestant said Mr Trump s3xually assaulted her during a meeting to discuss job opportunities at a Beverly Hills hotel.
She said he "began thrusting his genitals" as she fended off his advances.
The president-elect has dismissed the claims as "false and ridiculous" and vowed to sue the accusers for their allegations.