The 6-1 ruling gave US President George Bush a potential boost in his bid for re-election. Analysts believe Nader helped him win four years ago in the closely contested state.
Friday's decision met a Saturday deadline for mailing 25,000 ballots to overseas voters, most of them military personnel, and ended a confusing two weeks during which Nader was on and off the ballot.
"This is a case that should have been thrown out of the courts sooner," said Kevin Zeese, a spokesman for the Nader campaign.
Democratic National Committee Chairman Terry McAuliffe issued a statement on Friday saying: "The fact that Ralph Nader secured a place on the Florida ballot by means of the Pat Buchanan Reform Party speaks for itself. In state after state, Nader has become an extension of the Republican Party and their corporate backers."
The Florida Democratic Party and several individual voters challenged his certification.
The key legal challenge against Nader was the contention that the Reform Party was no longer a bona-fide national party and didn't nominate Nader in a national convention, as required by Florida law, but did it in a conference call three months earlier.
Officials with the party and Nader argued that the Reform Party convention might have been small, but that it had legitimately confirmed him as their presidential nominee.
As Green Party candidate in 2000
Nader won 97,000 Florida votes
"We are relieved with the Supreme Court ruling to enable our candidate to be on the November ballot," said Patrick Slevin, spokesman for the Reform Party of Florida.
"But we are also angry and disappointed we had to go through this process to begin with. This is a black eye for Florida, and it should have never have happened."
As the Green Party candidate in 2000, Nader won 97,000 Florida votes. Most Democrats and many Republicans agree that those votes cost Democrat Al Gore the presidency.
George Bush beat Democrat Al Gore by a mere 527 votes in Florida after the US Supreme Court put an end to recounts of votes in the state, handing the disputed election to Bush after a week-long stand-off.
Nader's standing could once again increase his chance of influencing the outcome of the presidential election.
"This is a case that should have been thrown out of the courts sooner"
Nader campaign spokesman
Known for his pro-Muslim stance - demanding a "viable state" for Palestinians and slamming Washington for using economic sanctions against Iraq - Nader could end up hurting the community rather than helping it.
If Muslims shift much of the overwhelming support they gave Bush in the last election to Nader, rather than Kerry, they may inadvertently help re-elect Bush.
Bush and Kerry are in a statistical dead heat, and Kerry cannot afford to lose any votes to Nader.
Nader and his running mate Peter Miguel Camejo are on the ballot in 36 US states.
Nader is planning a nine-city tour of Florida at the end of the month, starting in Jacksonville and working his way south to Miami.