English actress Alice, 42, gave birth to a little girl named Elsie Marigold in Los Angeles. The newborn joins the couple's four-year-old daughter Ella Betsi.
In a statement to E! News, Ioan, 39 and Alice said, "We are beyond thrilled to announce the arrival of our daughter, Elsie Marigold, on September 13. A little sister for Ella Betsi.
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"We both feel very grateful and very blessed."
The newborn is thought to have been due on 25 September, but decided to make her arrival a little early.
Fantastic Four star Ioan and Alice announced their second pregnancy in May, saying they were "thrilled" to be expanding their family.
The Welsh actor previously admitted that being a dad kept him on his toes. "She's very, very strong. She's like her mother," he said last year of daughter Ella. "I've got my work cut out for me at home."
Ioan and Alice — who played Esther in The Vampire Diaries — first met in 2000 while costarring in 102 Dalmations. Seven years later, they tied the knot in Mexico.
"Alice has been very patient with my procrastinating with my male ways," Ioan admitted during their engagement. "She did give me somewhat of an ultimatum — a gentle nudge — which was the best thing she could do really."