The actor and his wife Catriona McGinn welcomed daughter Lachlyn on Tuesday, 17 February.
Mark-Paul confirmed the news via Twitter, telling fans: "So in love with our new baby girl, Lachlyn! Born last night 2/17. Mum and baby are healthy and happy.
"Two boys, two girls. Too perfect!"
Mark-Paul and Catriona are also parents to son Dekker, one, while Mark-Paul has kids Ava, eight, and Michael, ten, from his marriage to ex-wife Lisa Ann Russell.
Among those sending well-wishes was his former Saved By The Bell co-star Mario Lopez, who tweeted: "Congrats to my boy [Mark-Paul] on the birth of his baby girl! So happy for you man! Can't wait to meet Lachlyn!"
Mark-Paul played Zach Morris in Saved By The Bell, while Mario Lopez starred as A.C. Slater.
They recently joined co-stars Tiffani Thiessen (Kelly Kapowski) and Elizabeth Berkley (Jessie Spano) for a spoof reunion imagining what Bayside High would have looked life if Zach had knocked Kelly up!
In real life, actress Tiffani is pregnant with her second child. She's already mum to daughter Harper.
Other Bell stars with kids include Mario, dad to daughter Gia and son Dominic, and Elizabeth, who is mum to son Sky.
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