His death follows the Nicaraguan government's confirmation last week that Rivera had been detained since 2023, after his family, the US government and representatives of the United Nations demanded proof he was still alive.
In a statement, Nicaragua's health ministry said Rivera died from bacteria generated by COVID-19.
Initial news reports of his death had elicited outrage from human rights groups who say he had been subject to arbitrary detention and was a victim of political persecution.
"If he is dead, it cannot be said that the cause was illness. The cause would be that he was in government custody in conditions of enforced disappearance for over two years, denied independent medical oversight. There is no other way to read this," Reed Brody, a member of the UN Group of Human Rights Experts on Nicaragua, said in a statement before Nicaragua confirmed Rivera's death.
The announcement from Nicaragua's government came hours after reports of Rivera's death by 100 per cent Noticias and Confidencial, two news outlets that were forced to leave Nicaragua several years ago in the face of government repression against the press.
Rivera's daughter Tininiska did not immediately respond to a text message requesting comment.
Rivera was arrested in September 2023 as part of a government crackdown on political dissent, according to human rights groups.
He was a sitting MP at the time, and his family has said the government never formally admitted he was being held and denied family members visitation rights.
Wednesday's confirmation of Rivera's detention was accompanied by photographs showing him bedridden, intubated and severely emaciated.
The interior ministry described his condition as "delicate, with mechanical ventilation through a tracheotomy and intravenous feeding" owing to "multiple organ failure, a cirrhotic liver and an active lung infection".
The US Bureau of Western Hemisphere Affairs denounced the statement, accusing the Nicaraguan government of attempting to conceal its role in the "cruel treatment and current condition" of Rivera.
Rivera had fought against Nicaragua's first Sandinista government (1979 to 1990) as a leader of the Misurasata militia alongside the Contra rebels.
His political party, Yatama, was initially an opponent of President Daniel Ortega but became an ally after Ortega returned to power in 2007.
In October 2023, the indigenous Yatama party said the government had banned it from running in elections.
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