Turkey has agreed to a total ban on capital punishment.
Turkey's prime minister says joining the EU is his top priority
Its envoy to the Council of Europe signed a European Convention protocol abolishing the death penalty in all circumstances, including during wars.
The Turkish parliament had already voted to abolish the death penalty in peacetime in August 2002.
The European Commission said Ankara's latest move was a "significant step on its way to becoming a fully fledged democracy".
Analysts say the signing of the protocol is part of an extensive programme of human rights reforms being undertaken as Turkey seems membership of the European Union.
The protocol must be ratified by the Turkish parliament.
The EU is expected to decide at the end of the year whether to open entry negotiations with Ankara.
Turkey's permanent representative to Council of Europe, Numar Hazar, signed Protocol 13 at a ceremony in Strasbourg, the Turkish news agency Anatolia reported.
Protocol 13 prohibits the death penalty in all circumstances, including in times or war and at times of danger of war.
European Commission spokesman Jean-Christophe Filori said "the Commission very warmly welcomes this initiative which very good news for human rights in Turkey".
Mr Filori said the move "represents a further significant step for Turkey on its way to becoming a fully fledged democracy fully respecting European standards in terms of human rights".
A moratorium on the death penalty had already been in place in Turkey since 1984.
The death penalty was replaced by life imprisonment without parole. The change saved the life of Abdullah Ocalan, former head of the Kurdish militant organisation PKK.
He was originally sentenced to death in June 1999 for his role in a 16-year guerrilla war against the Turkish authorities in which more than 30,000 people were killed.
The EU has praised Turkey's determination in passing key democratic reforms, but said implementation had been slow and uneven.
Some politicians in Brussels say Turkey's military still have too much say in running the country.
They also say the culture of government is very different to that of other applicant states, with a lack of accountability.
And in private, some EU officials are somewhat uneasy about letting a predominantly Muslim country join the club.