The 40-year-old actress – who has 13-year-old Kai with David Dugan and seven-year-old Stellan with husband Paul Bettany – welcomed daughter Agnes Lark into the world on May 31, her spokesperson has confirmed.
The baby was born at the couple's home in New York in a scheduled water birth.
Jennifer's representative said: "Everyone is healthy and doing well. The family is very excited."
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Jennifer – who married Paul in 2003 after two years of dating – revealed earlier this year she had suffered badly with morning sickness during her third pregnancy.
She said in January: "The first three months were very sketchy. You know how like you have your cell phone and you have to stand in one spot? It was like that with nausea. I had to stand in one spot, which happened to be in front of my refrigerator, which was open with me dipping pretzels in cream cheese and stuffing them in my mouth."
The 'Dilemma' star has previously highlighted the importance of being a mother, and said it made her change her life.
She said: "Becoming a mother has made all the difference in terms of learning to take responsibility for myself and my life.
"Parenthood changed the way I do everything. The biggest thing for me is wanting my kids to grow up safely and have happy lives. For me, that's enough."