Sampedro, who advocated a more humane economics based on ideas of solidarity, passed away overnight on Sunday and his remains were cremated on Tuesday morning, his widow Olga Lucas said.
"He had asked that his death be treated without a fuss. I would like him to be remembered for his vitality, his dignity and his fighting spirit," she told news radio Cadena Ser.
Born in Barcelona in 1917, Sampedro lived in Tangiers in northern Morocco with his family when he was a child.
He taught economics at Madrid's Complutense University as well as at universities in Britain and the United States.
"There are two types of economists: those that work so that the rich become richer and those like me, who work so that the poor become less poor," Sampedro once said.
His works include "The Etruscan Smile" (1985), "The Economic Forces of Our Time" (1967), "Economic Structure" (1969), and "Humanist Economics, Beyond Numbers" (2009).
When the grass-roots "indignant" movement emerged in Spain in 2011, with activists occupying squares across the country against the government's handing of the economy, Sampedro emerged as its standard bearer and took part in several rallies despite his advanced age.
"To consider money as a supreme good will lead us to catastrophe," he said in an interview published in left-leaning daily newspaper El Pais in June 2011.
Sampedro cemented his bond with the protest movement when he wrote the forward for French writer Stephane Hessel's protest essay book "Time for Outrage!".
Hessel died in February at the age of 95.
"I was also born in 1917. I am also outraged. I also experienced war. I also endured a dictatorship," Sampedro wrote in the forward to Hessel's book.
Sampedro became a member of the Royal Spanish Academy in 1990 and in 2010 the Spanish cabinet made him a member of the Order of the Arts and Letters for his "remarkable literary trajectory and his commitment to the problems of his time".
A year later, he was awarded the National Prize of Spanish Letters.