The actress – who played Niki Sanders and Tracy Strauss in the show – welcomed daughter Vivienne Margaret with husband Hayes MacArthur on Thursday, 15 January.
In a post to her official website, the star said: "It's a girl! We couldn't be more excited to share news of the arrival of Vivienne Margaret MacArthur on Thursday January 15th 2015. She came bearing gifts for her brother: Batman sheets and lots of Gummy Bears!!! 7.5 lbs of love."
Ali and Hayes are also parents to son Theordore, four.
38-year-old Ali can currently be seen alongside Sean Bean in TV spy drama Legends, which was recently renewed for a second series.
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