The 'Destiny's Child' singer, who has been dating her manager Tim Witherspoon on and off for four years, flashed a huge sparkler on her wedding finger during a birthday video message he posted for a friend on Instagram.
According to celebrity website Bossip.com, Tim popped the question to the 'X Factor' USA judge sometime last week but they are yet to make an official announcement.
In the video, 33-year-old Kelly is seen waving to the camera and inadvertently revealing the ring.
There were rumours in September that the couple had already tied the knot and kept it under wraps.
They added: "Kelly wants to keep her sexy singer image alive, so she's keeping everything a secret. Most people didn't even know she had a boyfriend."
There have also been reports that the couple recently had matching tattoos on their wedding fingers, which sparked speculation that they could soon be planning to get married.
The pair have kept their relationship very low profile, in contrast to her bandmate Beyonce's marriage to rapper Jay-Z, and they have avoided being photographed together at events.