The Since You’ve Been Gone star announced the news to her fans on Twitter, writing:
“Our baby girl River Rose Blackstock arrived on June 12th! Thank you everyone for all of your well wishes! Brandon and I are on cloud 9!! :)”
Kelly, 32, gave birth on Thursday and was quickly congratulated by a host of famous names, including Ellen DeGeneres.
She first announced her pregnancy in November and has previously spoken of wanting to start a family with Brandon – who has two children from a previous relationship – saying last year:
“[We want a baby] like now! Practice makes perfect! We do, we do. We're trying, trying, trying. We have two kids, and they're so great. And I never wanted kids before, but now I want, like, five.”
Congratulations!