His royal palace announced Rainier's death on Wednesday, nearly a month after he was admitted with a lung infection to a heart and chest clinic that overlooks Monaco's glittering, yacht-filled harbour.
He was Europe's longest-reigning monarch.
Rainier died at 6.35am (0435 GMT). Prince Albert, his son and heir, was at his side. Rainier's doctors called Albert a little before 6am to tell him that the end was near, the palace said.
Rainier met his wife, Grace Kelly, in 1955 when she was the 25-year-old star attraction of the Cannes Film Festival. Their marriage in 1956 put Monaco on the world stage.
He never remarried after her death in a car accident in 1982.
Albert, their only son, succeeds Rainier as Monaco's de facto ruler until a formal investiture ceremony, expected after a mourning period.
Albert had already taken over the royal powers - but not the throne - from his father last week after a royal commission decided that Rainier was too sick to rule.
Rainier married American actress
Grace Kelly(R) in 1956
The 47-year-old Albert, groomed for decades to succeed his straight-laced father, is well-travelled, multilingual and a five-time bobsledding Olympian.
In recent years, he has been Monaco's top ambassador. A confirmed bachelor, raised in the spotlight, he also has been seen as a sometimes reluctant heir.
Albert inherits a principality world-renowned for its casino and the annual Monte Carlo Grand Prix, but also eager to promote itself as a diversified, modern economy.
Monaco, no larger than New York's Central Park, is nestled on the Mediterranean coast between Italy and the French Riviera.
After assuming the throne in 1949, Rainier embarked on five decades of relentless expansion. Using landfill from the sea, Monaco expanded its territory by 20%.
Affectionately known as the "builder prince", Rainier oversaw the building of a new port, an artificial beach, a sparkling culture centre, an underground railway station, and a new breakwater project to allow large yachts and cruise liners to dock in the main harbour.
But his energies flagged with advancing age and illness.
Prince Albert is expected to take
over his father's throne
The leader of one of Europe's longest-ruling royal families, the Grimaldis, Rainier suffered recurring health problems in recent years.
The prince underwent heart surgery in 1999. In 2000, he had two operations, including a nodule removed from a lung, and was hospitalised in 2002 for fatigue and bronchitis. Recurrent chest infections put him in the hospital on numerous occasions thereafter.
Monaco has worked to overcome its image as a shady place for shady people.
It came under fire in 2000, and its relationship with Paris came under strain when a series of French official reports criticised its culture of banking secrecy and portrayed the principality as a haven for drug barons, money-launderers and tax dodgers.
Since then, Monaco has improved judicial cooperation and introduced comprehensive controls on dirty money, according to the global Financial Action Task Force on money laundering and terrorist financing.
But the seaside state continues obstinately to withhold information from foreign tax authorities about their nationals' offshore assets - earning it a place on a dwindling
Organisation for Economic Cooperation and Development blacklist of "uncooperative tax havens", alongside Andorra, Liberia, Liechtenstein and the Marshall Islands.