The Supreme Court ruled Monday in favor of a black Georgia death-row inmate who claimed that prosectors kept African Americans off the jury that convicted him of murdering an elderly white woman.

The court ruled 7 to 1 that Georgia prosecutors had violated the Constitution by improperly considering race when selecting a jury to judge Timothy Tyrone Foster. Chief Justice John G. Roberts Jr. wrote for the majority.

Justice Clarence Thomas, the lone African American on the court, dissented, saying that the evidence that prosecutors acted improperly was not strong enough to overturn Foster’s conviction.

The decision did not expand the law, but it laid bare a jury selection process that usually is opaque. It came about because Foster’s lawyers, nearly two decades after the teenager’s conviction in 1987, gained access to prosecutors’ notes under Georgia’s open ­records laws.

The names of the black potential jurors were marked with a “B” and highlighted in green. Their race on juror questionnaires was circled. All were at the top of a list labeled “Definite NOs” and each was compared with the others, according to the notes, in case “it comes down to having to pick one of the black jurors.”

In the end, they did not.

“The focus on race in the prosecution’s file plainly demonstrates a concerted effort to keep black prospective jurors off the jury,” Roberts wrote.

The decision sends the case back to Georgia, where Foster’s lawyer said his client deserves a new trial.

It was one of three cases the court decided on Monday in which litigants who had claimed discrimination prevailed. The court decided that federal employees alleging discrimination have longer to file suit — Thomas dissented from that ruling as well. And it upheld a lower court’s decision that a new congressional redistricting plan was warranted in Virginia because of racial gerrymandering by the General Assembly.

In selecting juries, potential panel members may be struck by lawyers on either side after questioning for a variety of “cause” reasons, such as whether they know one of the parties to the case or because they acknowledge that they could not be fair.

But each side also gets a number of peremptory challenges, which allow the dismissal of a juror for no particular reason.

In 1986, the Supreme Court ruled in a case called Batson v. Kentucky that it was unconstitutional to strike jurors because of their race. Later, the court said jurors cannot be rejected because of their sex.

But almost no one thinks the problem has been eliminated. Prosecutors and defense attorneys need only find a benign reason for dismissal, and studies and experience have concluded that only the most incompetent lawyer will fail to come up with a justification that a judge can accept.

Numerous studies have shown that prosecutors use their peremptory challenges to strike black potential jurors at a much higher rate than whites, because surveys show that African Americans are more skeptical of police and prosecutors. Criminal defense lawyers thus use their strikes on white potential jurors.

Foster’s lawyer at the Supreme Court, Stephen B. Bright of the Southern Center for Human Rights in Atlanta, said his client’s case is a textbook example.

Foster, then 18, was arrested in the killing of Queen Madge White, a 79-year-old widow and former elementary school teacher in Rome, Ga. According to prosecutors, “Foster broke into White’s home. He broke her jaw, coated her face with talcum powder, sexually molested her with a salad dressing bottle and strangled her to death, all before taking items from her home.”

Turned in by his girlfriend, Foster acknowledged the crime. But there were questions about his limited intellectual capacity and whether he had acted alone.

But a judge accepted the prosecutor’s reasons for striking the black potential jurors, and Georgia’s Supreme Court upheld ­lower-court decisions against Foster.

In a statement, Bright said the U.S. Supreme Court had no choice but to overturn those findings because of the unusual evidence discovered in the case.

But he added: “The decision in this case will not end discrimination in jury selection. Justice Thurgood Marshall said in Batson v. Kentucky that it would end only with the elimination of peremptory strikes. The choice going forward is between the elimination or reduction of peremptory strikes or continued discrimination.”

The court took a close look at how prosecutors selected the jury, and Roberts wrote that prosecutors’ reasons for excluding at least two of the potential black jurors were not believable. He used words such as “mischaracterized” and “nonsense.”

“Two peremptory strikes on the basis of race are two more than the Constitution allows,” Roberts wrote.

In his dissent, Thomas did not find the evidence as compelling as his colleagues. He said he would have left the matter with the state courts.

“It was the trial court that observed the [jury selection] firsthand and heard them answer the prosecution’s questions, and its evaluation of the prosecution’s credibility on this point is certainly far better than this court’s nearly 30 years later,” Thomas wrote.

He said the decision will lead to more defendants trying to reopen cases thought final.

“The court today invites state prisoners to go searching for new ‘evidence’ by demanding the files of the prosecutors who long ago convicted them,” Thomas wrote. “If those prisoners succeed, then apparently this court’s doors are open to conduct the credibility determination anew.”

Justice Samuel A. Alito Jr. agreed only that the lower court’s decision could not be affirmed, and he offered his view of how the Georgia courts could proceed, which would not necessarily mean a new trial.

The case is Foster v. Chatman.