Hisham Omeisy had spoken out against restrictions in rebel-held areas but had also criticised the actions of a Saudi-led coalition that has been fighting the rebels since 2015.
"The release of Hisham al-Omeisy today is a welcome step -- however, Huthi authorities must also immediately and unconditionally free all prisoners of conscience languishing in detention," Amnesty's Samah Hadid said.
Pictures circulating on social media showed Omeisy hugging his two young sons, thin but apparently unharmed.
The rebels sentenced Yemeni Baha'i Hamid Haydar to death earlier this month on charges of collaborating with Israel and forging official papers, Amnesty said.
The Baha'is believe that Bahaullah, who was born in Iran in 1817, was the most recent prophet sent by God. They are widely persecuted in the Islamic world.
Human Rights Watch has said it has documented 66 cases of arbitrary arrest by the rebels, who control Sanaa and much of the north of Yemen.
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