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.
More on this and other Suffolk stories.
"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.
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.