Police officers were called at 3.07am on Saturday to the Cause in Ashley Road, Tottenham, after a number of people became unwell.
A 21-year-old man was pronounced dead in hospital. His next of kin have been informed, police said.
The death is being treated as unexplained. A post-mortem examination will be scheduled in due course.
Two other men are now said to be in a stable condition after also falling ill.
Police believe they all had taken drugs, and do not believe they were known to each other.
No arrests have been made.
Superintendent Simon Crick, in charge of policing for Enfield and Haringey, said: “There is no safe way of taking illegal drugs, however the incidents overnight have caused us particular concern.
“We urge anyone who bought drugs in or around the nightclub to dispose of these substances as soon as possible.”
To report information about drugs and drugs supply call 101 quoting CAD 877/31Jul. You can also contact Crimestoppers anonymously on 0800 555 111.
Anyone who needs emergency medical attention should call 999.