Rowland announced her betrothal at a taping of Wednesday's edition of "The Queen Latifah Show," UsMagazine.com reported.
"He put a ring on it," Rowland said, holding up her left hand for the audience.
"It's been a while," Rowland added, without saying specifically when she got engaged. "We've kept it very, very low -- for years, to be completely honest."
The former Destiny's Child star went on to say her beau proposed to her via Skype when she was "in Bulgaria somwhere" and they were chatting by video conference.
"It was so late -- we were just staring at each other," she said, explaining he later cemented the deal by offering her a beautiful diamond ring.
"I have my best friend to ride through life with," she said of Witherspoon. "He really is my best friend."