Dimitris Koufontinas, a senior member of the November 17 terrorist group, returned to the Korydallos prison, west of Athens following a controversial two-day leave.
Koufontinas was jailed for life over the killings of 11 people including US, British and Turkish embassy staff.
The decision to temporarily allow him out of jail for the first time in 15 years, sparked uproar from the families of the group’s victims.
The far-left hitman, now 59, returned to Korydallos Prison around 90 minutes ahead of a required deadline, a justice source said.
His two-day leave sparked criticism from the US, Britain and Turkey.
US ambassador to Greece Geoffrey Pyatt said it was an insult to victims and their families.
“I add my voice to those from across Greece’s political spectrum deploring prison council decision to release a convicted terrorist, murderer & N17 leader,” Pyatt said on Twitter.
British ambassador Kate Smith likewise tweeted London’s “profound disappointment” and added that the embassy “shared” the pain of the victims’ families.
And the Turkish foreign ministry said the decision had displayed “tolerance to a bloodthirsty terrorist” in “sheer disrespect to the memory of our martyred diplomats.”
Koufontinas, who evaded police in 2002 for months by camping out on a secluded beach when the other members of the group were arrested, eventually turned himself in.
He later wrote a best seller on his life inside the extremist group.
Opposition parties have assailed the leftist government of Prime Minister Alexis Tsipras, accused by domestic critics of being soft on far-left militants, noting that Koufontinas had never shown remorse.