The five returnees — four from Pakistan and one from Egypt — were sent to the Turkish port of Dikeli on a ship operated by European border agency Frontex, after their asylum requests were rejected on appeal.
Under the controversial EU-Turkey deal struck in March, migrants arriving in Greece were expected to be sent back to Turkey if they did not apply for asylum, or if their asylum requests were refused.
In reality, the 500 would-be migrants who have been sent back since the deal came into force on March 20 had either refrained from requesting asylum, or withdrawn their applications, a source at the Greek ministry of migration said.
The five sent back on Thursday were the first returnees to have had their requests turned down.
In a desperate bid to avoid being sent back to Turkey, most arrivals since March 20 have applied for asylum, regardless of their status or where they came from.
The Greek teams working on refugee status determination have been overwhelmed with the sheer scale of the crisis on both the islands and the mainland.
The July 15 coup bid in Turkey has also caused additional delays to Greece´s efforts to return failed asylum seekers.
Around 13,000 migrants and refugees have been stuck on the Aegean islands since March 20, many of them on Chios and Lesbos, waiting to have their cases heard.
Another 56,000 migrants who arrived in Greece before the deal came into force, but after the closure of the so-called Balkan route, are stuck on the mainland.
Around this time last year, the Greek islands became the epicentre of the worst migrant crisis in Europe since World War II, with thousands of people desperately fleeing war and poverty landing on their shores each day.
The International Organization for Migration (IOM) said Thursday it has helped secure the voluntary repatriation of 4,285 people, mainly from Afghanistan and Iraq, since the start of the year.