Radio reports quoting family sources said Niarchos, 86, died on Monday after six weeks in intensive care at the Zurich cantonal hospital. He will be buried in St. Moritz, Switzerland, on Wednesday, they said.
Niarchos, a man of great energy and huge luck in business, was still skiing - his favourite sport - after the age of 80 but in recent years he had been in and out of hospital. The cause of his death was not given.
His personal life was as turbulent as the sea from which he made his fortune. He married five times and was fiercely competitive in everything he did.
He earned millions of dollars shipping crude oil around the world and spent much of his life competing for fame and fortune with his arch rival, Onassis, even marrying the shipowner's former wife.
Niarchos, who had made his home in Switzerland for years, attributed his success 60 per cent to energy and 40 per cent to luck.
He built one of the world's largest private tanker fleets, some four million tonnes, and weathered the slump that hit the world merchant fleet in the mid-1980s.
His jet-set lifestyle made his name synonymous with wealth. He entertained world leaders and socialites on his luxury yachts and private Greek island and collected racehorses and rare paintings.
Born in the Greek port city of Piraeus near Athens on 3 July 1909, he entered his family's grain business after earning a law degree at the University of Athens.
He set the course for his later fortune by convincing his family it would be more profitable to operate their own shipping fleet and to buy ships of the largest possible tonnage.