The complaint, filed by Andrea McNulty, claims the incident occured in July 2008. McNulty is reportedly an employee at Harrah's.
Roethlisberger, who has led the Steelers to two Super Bowl victories in five seasons with the team, is being represented by the law firm DNK Cornwell in the case.
Roethlisberger played in the Tahoe Celebs American Century Championship last weekend after the complaint was served.
"Ben has never sexually assaulted anyone; especially Andrea McNulty," the firm said in a released statement. "The timing of the lawsuit and the absence of a criminal complaint and criminal investigation are the most compelling evidence of the absence of any criminal conduct.
If an investigation is commenced, Ben will cooperate fully and Ben will be fully exonerated.
"Ben will not be baited into a public discussion about his personal life, but we will defend him vigorously. The limited value that Ms. McNulty derived from public disclosure of these viciously false allegations is the only value she will ever receive from Ben in this case."