The Pretty Little Liars star confirmed the news on Instagram on Friday (25Mar16), with a sweet black-and-white snap of his family of five.

"Nixie Barbara Lowe has arrived! We're now officially outnumbered and we couldn't be happier! #Blessed #Grateful," he captioned the photo which shows him alongside his wife and their daughters Mabel, six, Fiona, three, and new addition Nixie.

The 48-year-old actor also changed his Instagram profile picture to one of him holding his newborn daughter.

Chad, who is the younger brother of Hollywood star Rob Lowe, began dating Kim in 2007, and they welcomed first child Mabel in 2009. The couple wed in August 2010 in an intimate ceremony in Los Angeles, California.

The actor was previously married to actress Hilary Swank, from 1997 to 2007. Hilary famously forgot to thank Chad during her acceptance speech when she won her first Oscar for Best Actress in 2000 for Boys Don't Cry. However, after winning her second Oscar in 2005 for Million Dollar Baby, Chad was the first person she thanked.

The couple split in 2006, and their divorce was finalised in 2007.

Hilary also had reason to celebrate this week - her boyfriend, tennis coach Ruben Torres, has proposed. "Went on a hike and this happened!" she wrote on Instagram on Tuesday (22Mar16), sharing a photo of her with a sparkler on her engagement finger. "I am so happy to share with you all, the engagement to my dear Ruben."