The rocker is now a granddad to grandson Revel Young Ian, the first child for his daughter Pearl Aday and her husband Scott Ian.
Ian, star of band Anthrax, wrote a message on Twitter confirming the news and said: '[This is the] happiest day of my life.'
It's claimed Ian was planning to stay with his wife and not appear on a number of the band's tour dates in July so he could be with Aday when she gave birth.
However it is not known if the baby's recent arrival will mean he will return for the dates.