The second round of voting had been scheduled for Sunday, with polling stations abroad already open, the Reuters news agency reported.
The run-off would have pitted Calin Georgescu, a far-right, pro-Russian candidate, against pro-European Union centrist leader Elena Lasconi.
"The electoral process to elect Romania's president will be fully re-run, and the government will set a new date and ... calendar for the necessary steps," the court said in a statement, as cited by Reuters.
Photo: EPA/ROBERT GHEMENT
Georgescu, who had polled in the single digits before the first round of voting on November 24, unexpectedly surged to first place, raising suspicions about the integrity of the results.
Georgescu advocates ending Romania's support for Ukraine against Russia's invasion. His victory could disrupt the pro-Western stance of the EU and NATO member state, according to Reuters.