Democratic congressional leaders urged Wu to quit after the allegations against him emerged.
The (Portland) Oregonian reported Friday that a young woman, a Wu campaign-backer's daughter, left a message with the congressman's Portland office accusing him of "aggressive and unwanted sexual behavior" in California last fall.
Wu issued no statement but a party official told The Oregonian of his decision Sunday night. Wu has served seven terms.
"I call on the Ethics Committee to initiate an investigation into the allegations against Congressman Wu," House Minority Leader Nancy Pelosi, D-Calif., said Sunday. "With deep disappointment and sadness about this situation, I hope that the Ethics Committee will take up this matter."
Rep. Steve Israel, D-N.Y., chairman of the House Democratic campaign committee, said: "New allegations of unacceptable behavior by Congressman Wu are extremely serious and disturbing."
A Democratic county committee in Wu's district has called a no-confidence vote for Wednesday.