The baby, Eloise Joni Richards, joins big sisters 7-year-old Sami and 6-year-old Lola, Richards' two daughters with ex-husband Charlie Sheen.
Eloise Joni was named after Richards' mother Joni, who passed away in 2007.
Shortly after the news broke, Richards posted a message of appreciation to her fans on Twitter: "Thank you for all your sweet messages."
She also Tweeted: "The girls and I are over the moon and so in my dad, grandpa Irv ... xo."