The 27-year-old athlete – an Olympic, European and Commonwealth long-jump champion – announced the happy news on Monday, sharing an adorable photo that showed the newborn's tiny hand.
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Greg Rutherford and Susie Verrill have welcomed their first baby
"Happiest day of my life," proud Greg wrote alongside. New mum Susie posted the same picture, which she captioned, "And then there was three."
The couple have not officially revealed their child's gender or name – although Susie's brother seemed to let it slip in a post on Twitter.
"Congratulations to my sister @rugbycupcakes and @GregJRutherford on the birth of baby Milo! He's beautiful," Paddy Verrill wrote in a tweet, which has since been deleted.
The proud parents both shared this photo of their new arrival's tiny hand
Greg previously revealed to Parentdish that Susie's due date was 16 October – a few weeks after his sporting season ended, meaning the couple had some time to prepare for the new arrival.
"I just can't wait to be around baby," he said. "I think the first time baby recognises me and smiles is going to be one of the most amazing times in my life, to be honest.
"I also can't wait to play with baby. I was quite lucky, although my dad worked a lot, whenever I was free he would play sports with me and my brother, and I can't wait to do that with my child."