The ITF said in a statement that Mansur Osanloo's release was "conditional on his "good behavior" and the payment of a bond.
Osanloo is the the head of the Vahed Syndicate, a free trade union of Tehran's bus workers which he co-founded in 2005.
Osanloo, who the ITF says was detained and tortured after he founded the Vahed Syndicate, was re-arrested in 2007 and sentenced to five years in jail after being found guilty of "acting against national security" and "propaganda against the state."
In 2010, his sentence was extended by one year.
The ITS said in a statement that Osanloo's health had worsened in prison.