The boat from Turkey was spotted 35 kilometres off the Romanian coast in rough seas and was towed by the coast guard to the port of Constanta.
"Among the migrants there are children, but we are still counting," coast guard patrol spokeswoman Ionela Pasat told AFP, who said the nationality of the migrants is not yet known.
The Black Sea emerged as an alternative route to western Europe over the summer, with the arrival in Romania of more than 570 Iraqis, Syrians, Afghans, Iranians and Pakistanis in less than a month.
In September, 21 people were confirmed drowned and up to 15 more were missing after a fishing boat carrying migrants sank off Turkey's Black Sea coast. A further 40 were rescued.
EU member Romania is not part of the bloc's passport-free Schengen zone and until now has largely avoided the kind of influx of refugees and migrants seen elsewhere on the continent over the past few years.
Fleeing war and poverty, migrants have sought new routes to Europe since the EU reached controversial agreements with Turkey and Libya to try to stem the flow to Greece and Italy.