The 31-year-old's wife Jodi Albert, a singer in the girlband Wonderland, gave birth to a son named Koa yesterday (December 20).
"Our Little baby Boy "Koa" was born today at 6.10pm," he confirmed on Twitter. "@JodiAlbert_xxx is an amazing woman.Love them both so much.Both doing amazing,so proud x"
Egan's baby boy shortly began trending on the social networking site as fans commented on the couple's choice of name.
"Wow thanks guys," he wrote in response. "Amazing to see our little boy Koa is trending in the Uk! And he's only a few hrs old!!!"
Egan and Albert first announced the pregnancy back in July. The couple married in May 2009 after more than five years together.
Westlife are set to split next year after a farewell tour of the UK in May.
Watch Westlife in the video for 'Bop Bop Baby' below: