"I've decided to run as an independent candidate for president," not representing the Greens, he said on NBC television's Meet the Press programme on Sunday.
Nader is blamed by many Democrats for former Vice President Al Gore's achingly narrow defeat by George Bush in 2000.
Nader had convinced many Democratic voters then that there was little difference between Bush and Gore and he restated that viewpoint on Sunday.
"Both parties are flunking, Republicans with a D minus and Democrats with a D plus," Nader said. "It's time to change the equation."
Nader said he made his decision "after careful thought" amid warnings from Democratic leaders that his attempt might threaten the prospect of a Democrat defeating Bush in the November 2004 election.
"This country has more problems and injustice than it deserves," he said, complaining that there is "too much power and wealth in too little hands."
Nader has said he will not run again as a Green because that party's convention is not held until June, which he says he believes is too late to launch a presidential campaign.
"This country has more problems and injustice than it deserves... (there is) too much power and wealth in too little hands"
Candidate for US presidency
The question of whether Nader should run as a Green, however, had been a divisive one among many in the pro-environment group, with many opposing a Nader candidacy because he is not a declared member of that party.
One top US Democrat, New Mexico Governor Bill Richardson, expressed the despair felt by many in his party on Sunday about Nader's announced run.
"It's his personal vanity, because he has no movement, nobody's backing him. The Greens aren't backing him. His friends urge him not to do it. It's all about himself," Richardson told Fox News.