Dimitris Koufontinas, 59, a senior member of the November 17 extremist group, was granted leave from Korydallos Prison for the first time in 15 years.
Koufontinas, dubbed "Poison Hand" by Greek media and serving multiple life sentences, must report to a police station twice a day.
The former mathematician and beekeeper emerged smiling from the top security prison, greeted a few friends and made no comments to a media scrum gathered outside.
Koufontinas, who evaded police in 2002 for months by camping out on a secluded beach when the rest of the outfit were arrested, eventually turned himself in.
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He later wrote a bestseller on his life inside the extremist group.
Opposition parties blasted the leftist government of Prime Minister Alexis Tsipras, noting that the renowned killer had never shown any remorse for his actions.
"A man who murdered 11 people... should not be leaving prison," added fellow opposition leader Stavros Theodorakis.
The government said it was not the first to take such action. A previous conservative administration -- including Mitsotakis in the cabinet -- had given leave to another of the group's chief assassins, Christodoulos Xiros, in 2014.
Xiros promptly vanished and was on the run for several months before being re-arrested.