Lieutenant Jeff Goodwin says authorities received a call shortly before 10.30am on Saturday (Nevada time) about the alleged battery at the Palms Casino Resort.
Goodwin would not release any details about the allegation, saying the investigation is still going on.
Brown was freed from felony probation last year, more than six years after he attacked then-girlfriend Rihanna. The singer pleaded guilty to felony assault in that case.
Brown's lawyer and publicist did not immediately return messages seeking comment.