The Coronation Street and Mr Selfridge actress welcomed daughter Orla on Tuesday, 11 March.
It's her first child with husband Ryan Clark.
A thrilled Ryan described Orla's birth as "the best gift in the world", adding: "Absolutey smitten… and so proud of my wife."
Katherine, 34, announced her pregnancy in September 2013, just one month after tying the knot with Ryan in Las Vegas. They'd been dating for two years before eloping to Sin City for a wedding at the Little Church Of The West.
She left her role of Becky McDonald in Corrie in 2012.
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