He was hospitalised last Monday with respiratory problems related to Alzheimer's, his family said.
Suarez, who served as prime minister from 1976 to 1981, was recovering from pneumonia, but his general health declined in recent days.
The Spanish politician had not appeared in public since 2003.
King Juan Carlos praised Suarez as an "exceptional contributor" in bringing democracy to Spain and praised the late politician for defending "the unity and diversity" of the country.
The former prime minister's son, Adolfo Suarez Illana, told the public last Friday that the man who led Spain's first democratic government after the end of the Franco dictatorship was near death at a Madrid hospital.
Born on Sept 25, 1932, in the central town of Cebreros, Suarez is considered one of the politicians who played a decisive role in Spain's transition to democracy, following the 1939 to 1975 Franco dictatorship.
"My gratitude (to Suarez) is deep and permanent, and my pain today is great," King Juan Carlos said in a statement posted on his Web site.
Suarez played a role in "one of the most brilliant chapters in the history of Spain: the Transition led by the Spanish people," the king said.