WASHINGTON — President Bush overstepped his authority when he ordered a Texas court to reopen the case of a Mexican on death row for rape and murder, the Supreme Court said today.

In a case that mixes presidential power, international relations, and the death penalty, the court sided with Texas and rebuked Mr. Bush by a 6-3 vote.

The president was in the unusual position of siding with a death row prisoner, Jose Ernesto Medellin, a Mexican citizen whom police prevented from consulting with Mexican diplomats, as provided by international treaty.

An international court ruled in 2004 that the convictions of Medellin and 50 other Mexicans on death row around America violated the 1963 Vienna Convention, which provides that people arrested abroad should have access to their home country’s consular officials. The International Court of Justice, also known as the world court, said the Mexican prisoners should have new court hearings to determine whether the violation affected their cases.

Mr. Bush, who oversaw 152 executions as Texas governor, disagreed with the decision. But he said it must be carried out by state courts because America had agreed to abide by the world court’s rulings in such cases. The administration argued that the president’s declaration is reason enough for Texas to grant Medellin a new hearing.

Chief Justice Roberts, writing for the majority, disagreed. Justice Roberts said the international court decision cannot be forced upon the states.

The president may not “establish binding rules of decision that pre-empt contrary state law,” Justice Roberts said. Neither does the treaty, by itself, require individual states to take action, he said.

Justices Breyer, Ginsburg, and Souter dissented.

The international court judgment should be enforced, Justice Breyer wrote. “The nation may well break its word even though the president seeks to live up to that word,” he said.

Justice Stevens, while agreeing with the outcome of the case, said nothing prevents Texas from giving Medellin another hearing even though it is not compelled to do so.

“Texas’ duty in this respect is all the greater since it was Texas that — by failing to provide consular notice in accordance with the Vienna Convention — ensnared the United States in the current controversy,” Justice Stevens said.

Medellin was arrested a few days after the killings of Jennifer Ertman, 14, and Elizabeth Pena, 16, in Houston in June 1993. He was told he had a right to remain silent and have a lawyer present, but the police did not tell him that he could request assistance from the Mexican consulate.

Medellin, who speaks, reads, and writes English, gave a written confession. He was convicted of murder in the course of a sexual assault, a capital offense in Texas. A judge sentenced him to death in October 1994.

Texas acknowledged that Medellin was not told he could ask for help from Mexican diplomats, but argued that he forfeited the right because he never raised the issue at trial or sentencing. In any case, the state said, the diplomats’ intercession would not have made any difference in the outcome of the case.

State and federal courts rejected Medellin’s claim when he raised it on appeal.

Then, in 2003, Mexico sued America in the International Court of Justice in The Hague on behalf of Medellin and 50 other Mexicans on death row in America who also had been denied access to their country’s diplomats following their arrests.

A Harris County assistant district attorney who handles capital case appeals, Roe Wilson, applauded the Supreme Court decision. “This case has been in the court system a long time based on various issues, ” said Ms. Wilson, whose office prosecuted Medellin. “It was a heinous murder of two young girls who were only 14 and 16. It’s certainly time the case be resolved and the sentence be carried out.”

Medellin, who was 18 at the time of the slayings, turned 33 earlier this month. He’s now out of appeals and Ms. Wilson said her office will ask for an execution date once the Supreme Court resolves a separate case challenging lethal injections.

Mexico has no death penalty. Mexico and other opponents of capital punishment have sought to use the world court to fight for foreigners facing execution in America.

Forty-four Mexican prisoners affected by the decision remain on death row around the country, including 14 in Texas. One Mexican inmate formerly facing execution now is imprisoned for life because of the Supreme Court decision outlawing capital punishment for anyone under 18 at the time the crime was committed.

Mr. Bush has since said America will no longer allow the World Court to judge the consular access cases because of how death penalty opponents have tried to use the international tribunal.

The case is Medellin v. Texas, 06-984.