An incident at BP's Unity platform was reported to police at around 4.40am.
BP confirmed that "an incident" had occurred and the Health and Safety Executive (HSE) said it was liaising with police.
Police Scotland confirmed they had been "made aware of a death offshore".
"The incident occurred on the Unity platform and enquiries to establish the circumstances are on-going," the force said in a statement.
The platform is within the Forties pipeline system.
BP later confirmed that the man who died was an offshore worker employed by the firm Cape.
BP said it was "deeply saddened" and confirmed that the man's relatives have been told.
His age is not yet known and no other personal details have been released at this stage.
A BP spokeswoman said: "An incident occured around 4am this morning."
In a statement released later, the firm said: "BP is deeply saddened to confirm that an offshore worker has died following an incident on the Unity platform. The next of kin have been informed.
"The offshore worker was an employee of Cape. BP is working with Police Scotland, the HSE, Cape and other relevant organisations to support those affected and ensure that the incident is fully investigated.
"The company extends its deepest sympathies to the deceased's family and employers and will provide them with every support it can."
The Health and Safety Executive said: "We are aware and are liaising with Police Scotland."
A phoneline has been set up for any relatives' inquiries and can be contacted on 0808 1000 333.
The Unity platform is 97 miles off the coast of Aberdeen.
The death comes around six months after an oil worker died after falling from an offshore platform into the North Sea.
George Bartlett, 62, from Shotts, North Lanarkshire, fell during maintenance work on the Taqa-operated Harding platform, about 198 miles north east of Aberdeen, on February 27.
RMT regional organiser Jake Molloy said it was difficult to comment on the latest incident when details of what happened are not clear. All that is known is that the person had gone in the water at around 4am, he said.
Mr Molloy said: "It's coming so soon after the Harding when another man ended up in the water in the early hours of the morning so there's obvious questions about how he came to be there."