Fox is expected to be out for about two months, ESPN reported.
The 58-year-old coach, who was hospitalized Saturday, will have aortic valve replacement surgery early next week at a hospital in Charlotte, N.C.
Fox, whose team is off to a 7-1 start, knew he had a heart condition that eventually would require surgery, but had hoped to put it off until after the season, the sports network said.
Broncos offensive coordinator Adam Gase and defensive coordinator Jack Del Rio are expected to take on additional responsibilities during Fox's absence.
The NFL team released a statement saying Fox was taken to a Charlotte-area hospital "after feeling light-headed while playing golf on Saturday."
"I am doing fine," Fox said in a text message to ESPN analyst Chris Mortensen.
The network had initially reported its sources said Fox had experienced symptoms of a heart attack.
Fox coached the Carolina Panthers for nine seasons before being hired by the Broncos in 2012. He led them to a 13-3 mark and the top seed in the AFC playoffs during his first year in Denver.