The 28-year-old put her career on hold to have the child and announced the news on Twitter.
"Our beautiful baby boy Reggie Ennis-Hill was born yesterday. We couldn't be happier."
The gold medallist from London 2012 married her husband Andy in May 2013.
The Sheffield-born athlete's unexpected pregnancy meant that she was forced to skip Commonwealth Games in Glasgow later this month.
However, she did say in January that her long-term aim was to return to the sport to defend her Olympic title in Rio in 2016.
"My plans for 2014 have been completely turned upside down, but having had a couple of weeks to think about things from a career point of view I am 100% set on returning to full time athletics once our baby is born and go for a second gold medal at the Rio Olympics in 2016 - that challenge really excites me," she said back then.
"But in the short term I will make our baby a priority and enjoy the whole experience as much as possible."
- Sports & Recreation