The 57 year-old man, who has still to be named, was discovered by workmen on Sunday morning in the grounds of St Machar Academy in the city’s Bedford Avenue.
The area where the man was found has been cordoned off while enquiries continue to establish how he sustained his injuries.
A spokesman for the Aberdeen Division of Police Scotland said: “Police Scotland are continuing to make enquiries following the discovery of an injured man in the grounds of St. Machar Academy, Aberdeen.
The 57 year-old was found by workmen around 8.15am on Sunday morning. He was taken by ambulance to Aberdeen Royal Infirmary suffering from serious injuries.”
He continued: “It is unknown at this time how the male came by his injuries but he was found on an area of lower ground adjacent to Bedford Avenue. It is believed the incident leading to him being injured may have occurred in the early hours of Sunday.”
The spokesman added: ”We would like to speak to people who were in the Powis Crescent, Bedford Avenue and St Machar Drive areas of Aberdeen around 1.30am on Sunday morning. We are keen to establish how this man came about his injuries and ascertain whether the circumstances are criminal or accidental. Anyone with information regarding this incident is asked to contact Police Scotland on 101 or Crimestoppers on 0800 555 111 if you wish to remain anonymous.”