The 31-year-old, who is also mum to Kai, seven, Klay, four, and one-year-old Kit, announced the news on Twitter on Thursday 15 February.
“So happy to welcome our baby boy Cass Mac Rooney into the world weighing a healthy 8lb 10oz,” she wrote.
“He is beautiful.”
People had predicted the couple’s fourth child would also have a name beginning with a K, but the Rooneys have broken the tradition with the name Cass.
Fans seemed to love the name choice anyway.
Coleen announced she was expecting her fourth child with Wayne in August 2017.
“So happy,” she tweeted at the time. “Never denied the news, but I was always protecting it.
“Had scan and all checks are fine... baby number four is on its way.”
Congrats to the couple!