Emergency services were called to St Cecilia Street, in the Quarry Hill area, shortly after 9.30am this morning (Friday).
A sub-contractor, who was not working at the time, had what is thought to have been a heart attack.
He was given CPR and an on-site defibrillator used before he was taken to Leeds General Infirmary.
A spokesperson for Caddick Construction confirmed this afternoon that the man had died.
Wetherby-based firm Caddick is creating an entirely new neighbourhood in the area called SOYO (South of York Road) with apartments, bars, restaurants, a hotel and public realm spaces.
The spokesperson said: “Emergency services were called this morning to the SOYO site in Leeds after reports of a 35-year-old sub-contractor suffering a suspected heart attack.
“He was not working at the time and was at ground floor level.
“He was given CPR and an on-site defibrillator was used before he was taken to Leeds General Infirmary where he unfortunately passed away.
“His next of kin were informed and our thoughts are with them at this tragic time.
“The HSE (Health and Safety Executive) has been informed as a matter of course.”
A Yorkshire Ambulance spokespersons saiud: “We received an emergency call at 9.42am this morning to reports of a male suffering a medical incident at a construction site on St Cecilia Street in Leeds.
“We dispatched an ambulance, rapid response vehicle and our Hazardous Area Response Team to the scene and the patient was conveyed to Leeds General Infirmary.”