There were no immediate reports of damage or injuries.
The National Earth Physics Institute director Constantin Ionescu said the quake occurred today at 5:49 pm local time in the region of Vrancea, some 140 kilometres northeast of Bucharest.
It was the strongest earthquake to hit Bucharest this year. In March 1977 a temblor with a magnitude of 7.2 hit eastern Romania, killing more than 1,500 and demolishing large parts of the capital.