The actor – who plays Howard in the hit sitcom – and his wife Jocelyn Towne welcomed a son called Wilder Towne Helberg, a rep confirms to Us Weekly.
Little Wilder was born back on 23 April, but his parents kept his arrival a secret until now!
The couple are also parents to daughter Adeline, two.
Simon confirmed the pregnancy in January, after he walked the red carpet at the Screen Actors Guild Awards in LA with Jocelyn, who was sporting a prominent baby bump.
The 33-year-old previously told TV Fanatic of fatherhood: "it's just every day there's something completely new and amazing and you're watching sort of mirrored evolution in some way. It's actually pretty incredible. You're just watching this little, tiny blob become a person…it's an amazing time."
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Stars who welcomed babies in 2014:
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