The baby boy was born in Los Angeles and named Dekker Edward.
"We are overjoyed with the new addition to our family. We are madly in love with our little boy," Gosselaar said. "We are madly in love with our little boy. He is healthy and just perfect -- all we could ask for,"
This is Gosselaar's third child, after daughter Ava and son Michael with ex-wife Lisa Ann Russell. Dekker should make a smooth transition, as McGinn has gotten to know the elder children.
"Catriona's great with my kids," Gosselaar said when he and McGinn first got engaged. "My kids absolutely adore her."
The "Franklin & Bash" star wed McGinn in July 2012. They revealed they were expecting the following March.