However, such information must still be turned over by the prosecution before a trial.
Monday's ruling did not affect evidence that directly supports a person's innocence. That information must still be released to the defense by the prosecution before plea-bargaining, a plea or a trial.
In the case that brought Monday's ruling, immigration agents found 30 kilograms of marijuana in Anglea Ruiz's luggage.
U.S. prosecutors offered her what is known in U.S. District Court in San Diego as a "fast track" plea bargain.
That standard agreement asks a defendant to waive indictment, trial and appeal in exchange for a recommendation by the government for a lighter sentence -- two levels lower on the U.S. Sentencing Guidelines.
In Ruiz's case, that would have lowered her ordinary sentence, specified by the guidelines, from 18 months to 24 months to 12 months to 18 months.
In the standard plea bargain, the government specifies that it has turned over all information that might establish a defendant's innocence, and says it will continue to do so if more evidence is uncovered.
However, a defendant is also required to waive the right to receive information that might undermine the credibility of witnesses or informants against her.
When Ruiz refused to agree to that last provision, the plea bargain offer was withdrawn.
Ruiz was indicted, and though she had no plea bargain, she entered a plea of guilty.
At her sentencing, Ruiz asked the judge to lower her imprisonment by the same two levels she would have gotten in a plea bargain, but was refused.
A federal appeals court panel reversed, however, saying prosecutors should have made available any information impeaching a witness' credibility before plea-bargaining.
The justices heard argument in the case April 24, and reversed the lower court Monday.
Writing for the high-court majority, Justice Stephen Breyer said, "We have found no legal authority embodied either in this (Supreme) Court's past cases or in cases from other (appeals court) circuits that provide significant support for the (appeals court's) decision. To the contrary, this court has found that the Constitution ... does not require complete knowledge of the relevant circumstances ... but permits a court to accept a guilty plea ... despite various forms of misapprehension under which a defendant may labor."
Most of the rest of the justices signed on to Breyer's opinion. Justice Clarence Thomas wrote separately, but agreed with the judgment.
(No. 01-595, United States vs. Ruiz)