The woman dialed 911 to report the sexual assault had occurred when the bus was parked at a shopping center in Auburn, which is less than 20 miles (32 km) south of Seattle, local police said in a statement.
Nelly, best-known for the hit songs “Ride Wit Me,” “Hot in Herre” and “Dilemma,” performed at an amphitheater in Auburn on Friday night as part of a tour with country pop group Florida Georgia Line.
Patrol officers investigated the woman’s report, and Nelly was arrested before dawn, Auburn police said in the statement.
He was booked into the South Correctional Entity Regional Jail in Des Moines, Washington, on suspicion of sexual assault.
Auburn police had no further details to release, the statement said.
Nelly’s representatives did not immediately return emails seeking comment.