Polling stations opened at 09:00 GMT and will close at 22:00 GMT, with the result expected in the early hours of Sunday. According to Iceland's public broadcaster RUV, more than one in five of the country's under-400,000 registered voters had already cast their ballots in early voting before Saturday.
The ballot asks: "Should Iceland resume accession negotiations with the European Union?" Iceland is already a member of the European Economic Area and the Schengen area, giving it access to the EU's single market. Full membership would bring it into the customs union and eventually the euro.
Iceland's application to join the EU was well advanced when talks were put on hold in 2013. European Commission officials have indicated that the remaining negotiations could be finalised in one to two years. Of the 35 negotiating chapters, many were already provisionally closed.
The debate has been fought less over security—Iceland is a Nato member—than over the vital fishing industry and concerns about sovereignty. The centre-left government led by Prime Minister Frostadóttir had already planned to hold a referendum but accelerated the timetable in response to global uncertainty.