The 62-year-old man was on the Taqa-operated Harding platform about 198 miles north-east of Aberdeen when he fell into the North Sea during maintenance work at around 2.15am yesterday.
He was recovered from the water and airlifted to Gilbert Bain Hospital in Lerwick, Shetland, but later died.
The Health and Safety Executive is investigating the incident as it is being treated as a work-related death.
Production on the platform, which has a crew of 108 people, was shut down and an operation was under way last night to downman 74 non-essential workers from the installation.
A company spokesman said: "Taqa took the decision to downman out of consideration for the welfare of our people following this tragic incident.
"Our immediate thoughts are with the family and friends of our colleague at this extremely sad time.
"Taqa will conduct its own inquiry and is co-operating fully with all external investigations. The company is also currently offering its full support to the family of our colleague and will continue to do so for as long as required."
A Police Scotland spokesman said: "The area control room in Aberdeen received a report at approximately 3am of an incident on the Harding Platform in the North Sea, where a man had fallen into the sea.
"The investigation is currently being treated as a work-related death and Police Scotland are working to determine the full circumstances."