Turkey abolished the death penalty in 2004 as part of a campaign for EU membership. Turkish President Tayyip Erdogan has said he will approve its reinstatement if parliament submits such a proposal or if the measure is backed in a referendum.
"I am not of the opinion that the accession negotiations with Turkey should be stopped now," Juncker said in Berlin during a debate with students about the future of Europe.
European officials should still try to convince Turkey that it was in its own interest to adopt reforms and move towards Europe instead of turning away from the continent and its values, Juncker said.
"If Turkey will ever become a member state, I do not know," Juncker said.
He said he recently made clear in a long conversation with Erdogan that a reintroduction of the death penalty in Turkey would be a red line in accession talks.
"I told him: If you reintroduce the death penalty, then it's time to end," Juncker said.
Tensions between Turkey and the EU are high over rights and security issues, but the bloc depends on the help of NATO ally Ankara on migration and the conflict in Syria.
After meeting European Council President Donald Tusk and Juncker last week in Brussels, Erdogan was quoted as saying he had been presented with a new 12-month timetable for renewing ties.