The 40-year-old fell from the pavement outside GHQ in Edinburgh and tumbled down steps into the basement area directly outside the entrance to the club.
He suffered serious head injuries and was taken to the Edinburgh Royal Infirmary but was later transferred to the Western General Hospital where he remains in a serious condition.
The incident happened around 1.15am today (Mon).
Lothian and Borders Police are now appealing for witnesses who saw the man fall to come forward.
A police spokesman said: “The nightclub was busy with people entering and leaving at the time of the incident, and we are appealing for anyone who saw what happened to contact police immediately.
“Anyone with any information should contact Lothian and Borders Police on 0131 311 3131, or Crimestoppers in confidence or complete anonymity on 0800 555 111.”