Emad Baghi, the head of the Society to Defend the Rights of Prisoners, has been convicted on security charges, including acting against Iran's national security.
Radio Farda reported that Baghi's wife and daughter have also been sentenced to prison in connection with their participation in a conference abroad.
Their lawyer has said that they will appeal against the sentences.
The convictions are the latest in a series of rulings against activists and scholars accused of threatening Iran's national security.
Baghi has spoken out against the detention of U.S.-Iranian scholar Haleh Esfandiari, saying that she has not been given the right to defend herself.
(with material from agency reports)