The "Umbrella" singer, in a rare emotional TV interview set to air on Sunday detailing her thoughts about the highly publicized 2009 assault, told Winfrey that the days and weeks after incident left her confused and ashamed.
"It was embarrassing, it was humiliating," Rihanna told Winfrey in the interview, excerpts of which were released on Thursday. "I lost my best friend. Everything I knew switched, switched in a night, and I couldn't control that.
"It was a weird, confusing space to be in. Because as angry as I was - as angry and hurt and betrayed - I just felt like he made that mistake because he needed help. And who's going to help him?
"Nobody's going to say he needs help. Everybody's going to say he's a monster, without looking at the source. And I was more concerned about him," she added.
Rihanna, now 24, was left battered and bruised from the beating by Brown in Los Angeles on the eve of the Grammy Awards in February 2009. Brown, 19 at the time and a rising R&B singer, pleaded guilty to assault and was sentenced to five years probation, community service and domestic violence counseling.
He publicly apologized, but his career and public image took a dive. Brown's career has since recovered, and he won his first Grammy award earlier this year for the hit album "F.A.M.E".
Rihanna has rarely opened up emotionally in public about the events of that night.
Winfrey, who interviewed Rihanna in the singer's Barbados home, said in a promotion for Sunday's show that she had started with a fixed idea of Rihanna based on her provocative music videos and sexually charged lyrics, but her view quickly changed.
"I thought she was going to be kind of a badass, kind of a hard-edged rocker, pop woman. Nothing could have been further from the truth," Winfrey said.