"I shouldn't say, but I have to say it - I still don't have a smartphone," the 62-year-old told a news conference.
The light-hearted confession came as he helped launch the EU presidency of digital-savvy Estonia.
Mr Juncker joked that the country's Prime Minister Juri Ratas had "sent me, like in the 19th Century, a postcard inviting me to Tallinn".
According to EU sources, Mr Juncker's telephone of choice is an old Nokia mobile.
The EU chief is a former prime minister of Luxembourg, but said that with such technophobic tendencies, he "couldn't become prime minister of Estonia; this would be totally impossible".
Estonia is one of the world's most digitally-connected countries, and was the first to introduce online voting.
It hopes to push digital issues as part of its six-month stint as president of the EU, which begins on Saturday.
Tech matters will have to share space with the pressing issues of Brexit and migration, however.
Mr Juncker's admission comes 10 years after Apple introduced the iPhone, setting a global revolution in motion.
And yet, he is not the only political heavyweight resisting the march of technology.
German Chancellor Angela Merkel, who has led Germany since 2005, still does not have a Twitter account.
Jean-Claude Juncker, the president of the European Commission, has admitted that he does not own a smartphone.