Bryan Singer, who directed “Superman Returns” and several “X-Men” movies, was accused in a lawsuit on Thursday of sexually assaulting a 17-year-old boy at a party more than a decade ago.

In the lawsuit, Cesar Sanchez-Guzman said he met Mr. Singer for the first time at the 2003 party, which took place on a yacht in Seattle, when he offered to give him a tour of the boat. While showing him the master bedroom, according to the lawsuit, Mr. Singer shut the door behind them and demanded sex from Mr. Sanchez-Guzman.

After he refused, the lawsuit says, Mr. Singer pushed him on the bed and sexually assaulted him.

“Later, Bryan Singer approached Cesar and told him that he was a producer in Hollywood and that he could help Cesar get into acting as long as Cesar never said anything about the incident,” according to the lawsuit, which was filed on Thursday in a court in King County, Wash. “He then told Cesar that no one would believe him if he ever reported the incident, and that he could hire people who are capable of ruining someone’s reputation.”

A representative for Mr. Singer said he “categorically denies these allegations and will vehemently defend this lawsuit to the very end.” He added, “Once Bryan prevails, he will pursue his own claims for malicious prosecution.”