He won the first round of last year's race before the same court annulled the election.
The ruling by the Constitutional Court in the capital Bucharest, which was unanimous, came two days after the Central Election Bureau rejected Georgescu's candidacy for the May election.
It was not immediately clear on what grounds the court made Tuesday's decision, which is final.
Hundreds of Georgescu supporters who had gathered near the court, many waving Romanian flags, responded by chanting "Thieves" and "Freedom".
The bureau had cited in its decision on Sunday the Constitutional Court's ruling last year to cancel the elections after allegations of electoral violations and that Russian authorities had run a co-ordinated online campaign to promote Georgescu, who ran as an independent.
The Kremlin has denied this.
The decision to cancel the election was made two days before the December 8 runoff.
Georgescu had denounced his ban on Sunday as "a direct blow to the heart of democracy worldwide" and said "Europe is now a dictatorship, Romania is under tyranny!".
Georgescu supports the Romanian Orthodox Church and has sparked controversy in the past for describing Romanian fascist and nationalist leaders from the 1930s and 1940s as national heroes.
According to local media, he has praised Russian President Vladimir Putin as "a man who loves his country" but rejects claims he is pro-Russian.
The court's decision on Tuesday is likely to inflame tensions in the European Union and in the NATO member country, which has been gripped by a protracted political crisis since the election was cancelled last year.
The first round of the re-run is scheduled for May 4.
If no candidate wins more than 50 per cent of the ballots, a runoff will follow on May 18.
Georgescu remained voters' top choice for the May ballot, opinion surveys suggest.
Even with Georgescu out of the running, voters were expected to lean towards anti-establishment parties.
"I will vote for anyone new who isn't a part of this system and everyone I meet says the same, they are fed up with these same politicians," said 52-year-old taxi driver Ionel Popa.
with Reuters
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