The court said in a 5-4 decision that its ruling from June, which said juries and not judges should consider factors that can add years to defendants' prison sentences, also applies to the federal criminal sentencing guidelines.
In the June decision, the justices struck down a sentencing system used in Washington because it gave judges too much leeway in sentencing, but the high court on Wednesday stopped short of reversing the federal guidelines.
Justice Stephen Breyer said the federal sentencing system is in part invalid because it forces judges to use the guidelines, but that the system could be salvaged if judges used it on an advisory basis, the Associated Press reported.
About 64,000 people are sentenced in federal courts each year under a system that had been challenged as unconstitutional by two cases involving men convicted on drug charges in Wisconsin and Maine.
Wednesday's decision opens the door for thousands of claims by other defendants. Justice John Paul Stevens said not all of them will get new sentencing hearings. Judges must sort through the claims to determine which have appeals on the subject, he said, according to the AP.
The federal guidelines are meant to ensure that sentences do not vary widely from courtroom-to-courtroom. While juries determine a person's guilt or innocence, judges make factual decisions on factors that affect prison time, from the amount of drugs involved in a crime to the number of victims in a fraud.
The ruling was a defeat for the Justice Department, which had appealed to the Supreme Court in defense of the federal guidelines, saying the June ruling threw them into disarray.
Wednesday's ruling had the same combination of justices as the June ruling: Justices Antonin Scalia and Clarence Thomas, considered the court's most conservative members, joined liberal Justices Stevens, David Souter and Ruth Bader Ginsburg in voting for more jury participation.
The dissenting justices, Breyer, Sandra Day O'Connor, Anthony Kennedy and Chief Justice William Rehnquist, wrote, "history does not support a 'right to jury trial' in respect to sentencing facts.'"
Thousands of cases nationwide have been on hold pending the decision, according to Reuters.
"The whole federal criminal law system is operating in a state of suspended animation," said Seattle attorney Jeffrey Fisher, who argued the sentencing case in October at the start of the Supreme Court's nine-month term. People have been waiting "for the shoe to drop so we can start grappling with the hard issues in the aftermath," he said, reported the AP.
The cases are United States v. Booker (04-104) and United States v. Fanfan (04-105).