"The opening of Chapter 17 is part of the re-energising of Turkey's accession process as agreed at the EU-Turkey summit in November," Luxembourg Foreign Minister Jean Asselborn, whose country holds the EU presidency, told a press conference.
The opening of the new chapter -- on economic and monetary policy -- was a condition of an agreement between Turkish Prime Minister Ahmet Davutoglu and EU leaders at a special migration crisis summit last month.
But due to disagreements, over rights issues in particular, only one of them has been completed.
"Positive momentum in Turkey-EU relations continues with the opening of Chapter 17," Turkey's embassy to the EU said in a tweet.
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As well as the new chapter, the EU at last month's summit pledged USD 3.2 billion in aid for the more than two million Syrian refugees currently on Turkish soil.
The bloc also said it would end the visa requirement for Turkish visitors to the EU's passport-free Schengen zone.