Neither the arrest nor release of ex-judicial official Saeed Mortazavi have been explained by authorities.
Mr Mortazavi was jailed shortly after his political ally, President Mahmoud Ahmadinejad, accused Parliament Speaker Ali Larijani's family of attempting to use its prominence for financial gain.
The timing suggested it was linked to the claims and it was seen as an indication the President has lost favour with Iran's most powerful authority, Supreme Leader Ayatollah Ali Khamenei.
The squabbling among Iran's ruling elite appears to have intensified despite calls from Mr Khamenei for officials to avoid airing their disagreements in public.
Mr Khamenei has not commented on Mr Mortazavi's arrest or Mr Ahmadinejad's accusations against the Larijanis. However, he is likely to make a public speech in the coming days as Iran marks the 34th anniversary of the 1979 Islamic revolution this week.
Mr Mortazavi was suspended from his judicial post in 2010 over the torture deaths of three protesters in custody after the 2009 presidential elections, which the opposition said were rigged, bringing huge crowds into the streets.