prisoner of conscience
A person imprisoned for their beliefs, often used in human rights contexts.
He became a prisoner of conscience for seeking freedom of speech.
良心の囚人 is a more emotionally charged term specifically for someone imprisoned for their conscience, while 政治犯 is the neutral legal/political term.
Compound of 良心 (conscience) + の (possessive particle) + 囚人 (prisoner). The term is a direct translation of the English 'prisoner of conscience', popularized by Amnesty International.