Joan Bates and her late husband Major Roy Bates moved to an anti-aircraft fortress just outside British territorial waters almost 50 years ago.
They declared it an independent state and gave themselves the titles "Prince" Roy and "Princess" Joan of Sealand.
She is survived by their son, Michael.
Known as the "Prince" of Sealand, he has controlled Sealand, seven miles off the coast, since the death of his father in 2012.
Over the years, Sealand, a 10,000 sq ft platform, has encountered armed attacks by pirates and government bids to shut it down.
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"Prince" Michael of Sealand said: "My parents will always be remembered for shaking up the establishment with pirate radio, declaring Sealand's independence and confronting the Royal Navy and other foreign governments."
In 1978, three foreign men landed at Sealand by helicopter and overpowered Michael to claim the 'principality' as their own.
Mr Bates soon arrived, took two of the men hostage and regained control of the fort.

The tanker hit the central reservation halfway between junction 32 for Coryton and junction 33, Cardiff West, at about 17:45 GMT on Monday.
The westbound carriageway was closed from junction 30, Cardiff Gate, and one lane was also closed eastbound.
One person has been taken to the Royal Glamorgan Hospital in Llantrisant. Their condition was not known.
The Welsh Ambulance Service said other people involved in the crash were not injured.
There were long queues in both directions following the crash, but traffic has since eased. Diversions remain in place.
South Wales Police advised motorists to avoid the area for the rest of the night.
The Welsh Ambulance Service sent two rapid response vehicles, one ambulance and a doctor but an air ambulance could not be sent due to the weather conditions.
Two fire engines and one rescue tender from South Wales Fire and Rescue Service remained at the scene late on Monday evening to assist police.
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A woman who helped establish a self-proclaimed independent state on a former military platform off the Suffolk and Essex coast has died aged 86.
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One person has been taken to hospital after a serious crash on the M4 in Cardiff involving a tanker and a van.