Mousavi and his wife Zahra Rahnavard "were examined by a doctor at their home... and their general state of health is good," added the Kaleme site, close to the former prime minister-turned-dissident.
Mousavi, now 79, along with Mehdi Karroubi, 83, were reformist candidates in the controversial election of 2009, which was won by hardliner Mahmoud Ahmadinejad.
They claimed the vote was rigged, triggering months of mass protests dubbed the "Green Movement".
Hundreds of thousands took to the streets, particularly in the capital Tehran, in the biggest challenge to the system since the Islamic revolution of 1979.
Mousavi and Karroubi were placed under house arrest without trial in early 2011, along with their wives.
Iran's supreme leader Ayatollah Ali Khamenei called their opposition movement "sedition" and repeatedly demanded the two men repent before receiving any pardon.
Iran has been battling the Middle East's deadliest novel coronavirus outbreak since February.
It has recorded almost 41,500 deaths out of 762,000 cases, according to official figures considered to be underestimated, even by Iranian officials.
Authorities are considering imposing new nationwide anti-virus restrictions from November 21, after reporting a record 12,453 cases on Sunday, according to the health ministry.
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