A Socialist, he returned as PM in the early 1980s and served as president between 1986 and 1996.
He had been admitted to hospital in Lisbon last month following a "general worsening of his health", doctors said.
He played in a key role after the 1974 Carnation Revolution, a military coup that put an end to 48 years of dictatorship.
A fierce critic of the junta that ruled Portugal for the next two years, Mr Soares in 1976 became the first post-war democratically elected prime minister.
More about:
-1780923278.jpg&h=190&w=280&zc=1&q=100)
