Cuban dissident Miguel Valdes Tamayo has died of a heart attack, aged 50.
Fellow dissident Maria Beatriz Roque announced Mr Valdes' death
He was among a group of 75 dissidents jailed in 2003 after being accused of conspiring with the US - he was released on health grounds in 2004.
His death in the capital, Havana, was announced by fellow dissident Maria Beatriz Roque.
She said Mr Valdes had been refused permission to seek treatment abroad. "It was a cruel action by the Cuban government," she told AFP news agency.
Mr Valdes - who always denied the charges - had been sentenced to 15 years in prison.
The Havana authorities have already released 16 of those arrested in 2003.
The imprisonment of the 75 activists in 2003 triggered international condemnation.