It is understood that he was hit by the boom of a mini crane that he was directing near the north tower. He suffered severe blood loss and was taken off site, but he could not be resuscitated.
Another man was also taken to hospital with injuries described as minor.
A Police Scotland spokesman said: “We are currently working alongside our partner agencies to investigate an incident on the Queensferry Crossing bridge that resulted in a man sustaining fatal injuries.
“The call was received around 12.20pm and sadly, the man was pronounced dead a short time later. Another man on the bridge also sustained minor injuries.”
The dead man was later named as John Cousins.
The Unite union issued a statement saying that the worker was employed by the Forth Crossing Road Consortium and was working a spider crane when the incident occurred. "The 60-year-old man is understood to have been hit by the moving boom shortly after midday on the deck of the north tower," it said. "His death is thought to be the first fatality at the site where work began in 2011."
Harry Frew, regional secretary for construciton union Ucatt, said: “Ucatt sends its sincere condolences to the family of the worker who has suffered this tragic accident. It is impossible to imagine how they must be feeling.
UCATT will be doing everything it can to find out how and why this accident occurred and to ensure that similar fatalities are prevented in the future.”
The 2.7km bridge is being built by the Forth Crossing Bridge Constructors (FCBC) consortium, comprising Hochtief, Dragados, American Bridge and Morrison Construction under a £790m contract.