The 28-year-old, who won heptathlon gold at London 2012, announced on Twitter that her son Reggie was born on Thursday.
She said: "Our beautiful baby boy Reggie Ennis-Hill was born yesterday. We couldn't be happier."
Ennis-Hill, who is married to husband Andy, was the poster girl of London 2012.
She revealed in January she was pregnant, ruling her out of the Commonwealth Games in Glasgow which get under way next week.
She said at the time she was "100 per cent set on returning to full-time athletics" after the birth and determined to "go for a second gold medal at the Rio Olympics in 2016".
Congratulations were quick to flood in on Twitter from fellow athletes, including Katarina Johnson-Thompson, who is the favourite for the heptathlon title at Hampden Park in the absence of her compatriot.
Johnson-Thompson, the 21-year-old from Liverpool, has spoken of her excitement at going head to head with Ennis-Hill when the Sheffield athlete returns to the sport.
Ennis-Hill's last heptathlon was that famous one at the Olympic Stadium two summers ago, having missed last year's World Championships with an Achilles injury.