Kimberley, 32, is expecting a child with her long-term boyfriend Justin Scott later this year and announced the news on Twitter this afternoon.
The Girls Aloud star told her followers: "Justin and I are so happy to let you all know we are having a baby!!!
"Couldn't wait to share our lovely news with you all." Kimberley also posted icons of a baby and baby bottle.
Kimberley is the second Girls Aloud star to become a parent, as Nadine Coyle gave birth to a baby girl earlier this month.
Cheryl Cole has since taken to Twitter to express her relief at the news being made public, as she had been struggling to keep Kimberley and Justin's secret!
She tweeted: "@KimberleyJWalsh FINALLY, I thought at one point I was going to burst!"
Last week, Kimberley was pictured on the red carpet at the Brit Awards in London, but her flowing dress didn't give away any hints of a baby bump!
Kimberley has been dating Justin for more than 10 years and has spoken openly about how broody she has felt in recent years.
Last month, Kimberley shared a gorgeous candid picture of herself while cradling her baby nephew.
Before his birth, she had told Reveal: "I have got so many babies around me at the moment - friends, family and my sister is expecting again, so I'm going to be an aunty for the second time.
"I love kids so it's something I'm definitely going to do in the future."
Stars send their congrats to Kimberley Walsh and Justin Scott: