The 31-year-old singer announced the happy news Saturday via Twitter.
“Our baby girl River Rose Blackstock arrived on June 12th!” Clarkson wrote.
“Thank you everyone for all of your well wishes! Brandon and I are on cloud 9!!” she added.
Clarkson revealed that she was pregnant last November: “I’m pregnant!!! Brandon and I are so excited! Best early Christmas present ever.”
Later, in January, she announced the gender of her baby.
“We just found out we’re having a girl officially! I knew it! Only a girl could cause this much drama with all this vomiting ha!” Clarkson wrote then.
Prior to finding out that she was pregnant, the singer opened up about becoming a mother in an interview with Jay Leno.
“We have two kids. And they’re so great,” she told Leno. “I never wanted kids before, but now I want, like, five.”
The “American Idol” winner married Brandon Blackstock last October. This is the first child for the singer, and the third for her husband, who has two children, Savannah, 11, and Seth, 7, from a previous marriage.