"Executing the death penalty is incompatible with membership of the Council of Europe," the 47-member organisation, which includes Turkey, tweeted a day after President Recep Tayyip Erdogan said his government would ask parliament to consider reintroduction.
"Soon, soon, don't worry. It's happening soon, God willing," Erdogan told supporters in Ankara on Saturday as crowds chanted: "We want the death penalty!"
Erdogan, who did not specify a timetable, said his government would take the proposal to parliament, which said he was sure would approve it, allowing him to ratify the proposal.
Turkey abolished capital punishment in 2004 as the nation sought accession to the European Union.
Austrian Foreign Minister Sebastian Kurz added to the Council's warning, denouncing Turkey for considering a move that would "slam the door shut to the European Union".
"The death penalty is a cruel and inhumane form of punishment, which has to be abolished worldwide and stands in clear contradiction to the European values," Kurz told the Austrian Press Agency.
Ankara formally launched its EU membership bid in 2005. Since then, the bloc has opened 15 negotiating chapters out of 35 required to join, but to date only one has successfully been completed.
The issue has been back on the agenda following the coup, since when Ankara has fired thousands of civil servants and made some 35,000 arrests of presumed coup sympathisers.
Turkey's official gazette said earlier Sunday that the authorities had fired 10,131 additional government employees, mainly from the education, justice and health ministries as part of its post-coup clampdown.
The government also announced the closure of 15 pro-Kurdish and other media outlets.
Council of Europe secretary general Thorbjorn Jagland had already warned Ankara off capital punishment in August, noting the European Convention on Human Rights, which Turkey has ratified, clearly excluded it.
The Convention, signed in 1983, excludes capital punishment except in time of war or imminent threat of war and a 2002 protocol ended the time-of-war proviso.