The 19-year-old R&B singer appeared in a Los Angeles courtroom Thursday alongside his attorney.
He entered his plea in a soft voice while his mother sat in the first row, red-eyed.
Brown was charged in March by Los Angeles County prosecutors with felony assault likely to cause great bodily injury and making criminal threats. He was arrested nearly a month earlier after police said he was involved in an early morning altercation with a woman who identified the "Run It!" singer as her attacker.
Court records list Brown's victim as "Robyn F." Rihanna's real name is Robyn Rihanna Fenty.
The 21-year-old "Umbrella" singer was represented by attorney Donald Etra
If convicted, Brown could be sentenced to probation or nearly five years in prison.