Using the code name Donna she helped opponents of her brother's communist rule escape execution and imprisonment.
The details of her past have come to light in her memoirs, which were published in exile yesterday.
Revealing what the publishers called a closely guarded secret kept hidden for four decades, Juanita Castro described in the book how she was recruited by the US Central Intelligence Agency in Havana two years after the 1959 Revolution led by her brothers, which she initially supported.
There was no immediate reaction to her revelation from US authorities or the Cuban government, which routinely dismisses critics as mercenaries in the pay of Washington.
Juanita Castro, 76, broke publicly with the Cuban government led by her brother Fidel Castro in 1964 after leaving Cuba for Mexico. She went into exile in Miami and has remained a firm critic of communist rule in Cuba.
In the memoirs titled 'Fidel and Raul, My Brothers, the Secret History', told to Mexican journalist Maria Antonieta Collins, she says she quickly became disenchanted with Fidel Castro's rule over the Caribbean's largest island because he increasingly persecuted opponents and turned to communism.
She says Fidel Castro 'betrayed' her and other Cubans by abandoning the nationalist democratic revolution he had promised and imposing a one-party Marxist state on Cuba.
Juanita Castro wrote she was recruited to be a clandestine CIA operative by her friend Virginia Leitao da Cunha, the wife of the Brazilian ambassador to Cuba, who in 1958 sheltered her and other revolutionary followers of Castro during the armed struggle to topple dictator Fulgencio Batista.
She said that at a meeting with an American CIA officer 'Enrique' in a Mexico City hotel in 1961, she was given the code name Donna and codebooks to use in Cuba with a short-wave radio to receive instructions from her CIA handlers.
Former leader Fidel Castro, 83, who last year handed over the presidency of Cuba to his younger brother Raul, 78, for health reasons, has long considered the CIA his arch-enemy. He says the US spy agency was behind most of the 600 or so assassination plots he claims were made against his life.
In her memoirs, Juanita Castro wrote she agreed to work for the CIA under the noses of her brothers on the condition that she was not asked to participate in any violent acts against them or any other member of their government.
