The 89-year-old was admitted to the Quiron Hospital in Barcelona where he was being treated for heart problems.
The Spaniard was widely regarded as the most powerful man in sport when he headed the IOC from 1980 to 2001.
His successor, Jacques Rogge, said: “I cannot find the words to express the distress of the Olympic Family.”
Full story: BBC Sport – Ex-Olympic chief Juan Antonio Samaranch dies