The 62-year-old man was on the Taqa-operated Harding platform about 198 miles (320km) north east of Aberdeen when he fell into the sea during maintenance work at around 2.15am today.
He was recovered from the water and airlifted to Gilbert Bain Hospital in Lerwick, Shetland, but later died.
Last night, after the Health and Safety Executive investigation got under way, the oil platform was shut down.
Non-essential staff are being taken off the installation.
A spokesman for operators, Taqa, said: "Production has been shut down on the platform and an operation is under way to downman 74 non-essential personnel from the installation.
"Taqa took the decision to downman out of consideration for the welfare of our people following this tragic incident."
The platform has a crew of 108 people and all other workers have been accounted for, Taqa said.
A company spokesman said: "Taqa regrets to confirm there has been a fatality on the Harding platform in the North Sea.
"A Taqa employee fell overboard during a maintenance activity on the platform. He was recovered from the sea by a support vessel and transferred to Gilbert Bain Hospital in Shetland by helicopter. The next of kin has been informed.
"Our immediate thoughts are with the family and friends of our colleague at this extremely sad time. Taqa is providing its fullest support to all those involved.
"All other 108 people on board the Harding platform have been accounted for and are safe."
A Police Scotland spokesman said: "The area control room in Aberdeen received a report at approximately 3am today of an incident on the Harding Platform in the North Sea, where a man had fallen from the platform into the sea.
"As a result of the incident, a 62-year-old man was flown to hospital in Lerwick but has subsequently died.
"The investigation is currently being treated as a work-related death and Police Scotland are working with partners, carrying out a joint investigation with the Health and Safety Executive and under the direction of the Crown Office and Procurator Fiscal Service to determine the full circumstances."
Scottish First Minister Alex Salmond expressed his condolences to the man's family.
Mr Salmond told MSPs at Holyrood: "Sadly, I should tell the chamber we've had confirmation this morning a 62-year-old man fell from the Taqa Harding platform during maintenance activity in the North Sea.
"He was transferred to Gilbert Bain Hospital but unfortunately has passed away.
"I know the whole chamber will join me in expressing our sincere condolences to his family, friends and colleagues at this distressing time."