The 32-year-old took to Twitter to break the news, posting a snap of her stunning ring.
It comes just months after she and Justin welcomed their fist child together, son Bobby.
A source told the Mirror that the couple plan to get married next year.
Kimberley's former band mate and bestie Cheryl Fernandez-Versini retweeted the picture of the ring, adding a love heart emoticon.
Justin, 31, and Kimberley have been together for 10 years, and earlier this year she confessed she hoped it wouldn't be long before he popped the question.
She told a British magazine: "Maybe Justin will ask when I don't expect it!"
Kimberley recently opened up about struggling to breast feed Bobby, admitting she didn't it would be so uncomfortable.
She said: "I've always thought breast-feeding would be the easiest thing in the world and it's only when you speak to other people who tell you they also found it painful and hard that you realise it's not just you."