The rapper, 25, shared the news on Instagram alongside a photo of herself heavily pregnant surrounded by flowers.
The Bodak Yellow star told fans her daughter was born four days ago on 7 July.
Her husband, Migos rapper Offset – who she secretly married in September – also shared the news to his 7.5 million followers.
Earlier this year, Cardi B made an appearance on the Howard Stern Show and revealed she was expecting a girl.
She also told the host that Offset, 26, had chosen their daughter’s name.
It’s been a big year of announcements for the rapper.
In June, she told fans on Instagram she had married in secret because “there are moments that I want to keep for myself”.
She added: “Our relationship was so new breaking up and making up and we had a lot of growing up to do but we was so in love we didn’t want to lose each other, was one morning in September we woke up and decided to get married.”
She said she did not wear a wedding dress, wear make-up or have a wedding ring at the low-key ceremony.