Brown, 19, entered the pleas during his arraignment in Los Angeles Superior Court.
Rihanna’s attorney, Donald Etra, told reporters after the hearing that Rihanna hoped a plea bargain could be struck but that she would testify against Brown if required. Brown and his attorney Mark Geragos did not speak to reporters.
Judge Patricia Schnegg set an April 29 status hearing for Brown.
Los Angeles prosecutors have said Brown attacked Rihanna, 21, during a roadside dispute in his rented Lamborghini last February. A photo of Rihanna’s bruised and battered face was leaked onto the Internet.
Brown later issued a statement saying he was "sorry and saddened" about the incident, and was seeking counseling.
The formal charges accuse Brown of assaulting Rihanna "by means of force likely to produce great bodily injury" and threatening to commit a crime "which would result in death and great bodily injury." He could face up to four years in prison if convicted.
A police affidavit said Brown shoved Rihanna’s head against the passenger window, repeatedly punched her in the face, bit her ear and finger, and put her into a headlock. At one stage, the affidavit said, Brown told Rihanna: "I’m going to kill you."
Brown broke onto the Billboard charts in 2005 at the age of 16 with his single "Run It!" and sold 2 million copies of his self-titled debut album.
Barbados-born Rihanna also released her debut album in 2005 and had a No. 1 hit single with "S.O.S." less than a year later. She followed with three more No. 1’s — "Umbrella," "Take a Bow" and "Disturbia."