Vilma Espin, wife of Cuba's acting president Raul Castro, has died in Havana, aged 77, state TV reports.
Espin rejected her wealthy upbringing to join the revolution
She was a key figure in the Cuban revolution and the long-standing head of the Cuban Women's Federation, which works to advance women's rights.
Born into a wealthy family, she fought as a guerrilla alongside Fidel Castro and his younger brother Raul in the Sierra Maestra mountains.
She married Raul in early 1959 and was often described as Cuba's "first lady".
Espin reportedly died after a long battle with illness.
The Cuban authorities have announced an official mourning period, which will last until 2200 on Tuesday (0300 GMT Wednesday), with national flags on all public buildings and military bases being lowered to half mast.
There will be gatherings in her honour in the capital Havana and her hometown, Santiago.
"Her name will be linked eternally to the most significant achievements of Cuban women through the Revolution," a government statement, quoted by the Associated Press, said.
The daughter of an executive at the Bacardi rum distillery, Espin grew up in comfort in the eastern town of Santiago.
She was one of the first Cuban women to earn a degree in chemical engineering and did post-graduate studies at the Massachusetts Institute of Technology.
However, she turned her back on her upbringing in the 1950s, joining the armed struggle against right-wing dictator Fulgencio Batista and adopting the nom de guerre Deborah.
She married Raul in 1959 after Batista was forced to flee and the rebel fighters made their triumphant entry into Havana.
The couple went on to have four children.
In 1960 she founded the Cuban Women's Federation, a mass organisation with the objective of achieving full equality for women, which currently boasts 85% of Cuba's women as members.
Espin was a member of the Communist Party's Central Committee since its creation in 1965.
Her husband Raul has been serving as Cuba's temporary president since July 2006 when his brother Fidel underwent gastric surgery.