The Constitution "allows the president to execute the laws, not make them," Chief Justice John Roberts wrote in a rebuke of the president in a case that mixed presidential power, international relations and the death penalty.
Justice Samuel Alito, Bush's other nominee to the high court, signed onto Roberts' opinion.
The justices by a 6-3 vote said Texas also could ignore an international court ruling in favor of granting a new hearing to Jose Ernesto Medellin, facing the death penalty for killing two teenagers nearly 15 years ago.
Justice Stephen Breyer, in dissent, said the decision calls into question U.S. obligations under international treaties and makes it "more difficult to negotiate new ones."
The president was in the unusual position of siding with 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 the United States 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.
Bush, who oversaw 152 executions as Texas governor, disagreed with the decision. But he said it must be carried out by state courts because the United States 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.
White House press secretary Dana Perino said Bush was disappointed with the decision and is now reviewing it to see how it might impact international relations.
But, she said, "while we urged a different result, we respect the court's decision and will abide by it."
She noted that the administration's position in the case was focused on the authority it believed the president has to compel a state to comply with international agreements. "The argument of the United States in this case in no way condoned or defended the heinous crime," Perino said.
A Texas prosecutor whose office won a conviction against Medellin said she would ask for an execution date to be set as soon as the high court resolves a separate case over a challenge to lethal injection procedures. The court also could dispose of the cases of seven other Mexicans on death row in Texas as early as Monday.
Roberts, in the unfamiliar role of limiting presidential power, 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," Roberts said. Neither does the treaty, by itself, require individual states to take action, he said.
Breyer, joined by Justices Ruth Bader Ginsburg and David Souter, said the international court judgment should be enforced. "The nation may well break its word even though the president seeks to live up to that word," he said.
Justice John Paul Stevens did not sign onto the majority opinion but agreed with the outcome of the case. Stevens 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," Stevens said.
Justices Anthony Kennedy, Antonin Scalia and Clarence Thomas also joined Roberts' opinion.
Some legal commentators said the decision could have far-reaching implications for other international agreements involving the United States if they cannot be enforced within the United States.
Harold Koh, dean of the Yale Law School, said it could "disrupt a web of international relationships that turn on international dispute resolution." Koh filed a brief in support of Medellin.
But Texas Solicitor General Ted Cruz, who argued the state's case at the high court, said the decision "categorically prohibits foreign courts from undermining American sovereignty and independence."
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.
Mexico, which has no death penalty, then sued the United States in the world court in 2003. Mexico and other opponents of capital punishment have sought to use the world court to fight for foreigners facing execution in the U.S.
Forty-four Mexican prisoners affected by the decision remain on death row around the country, including 14 in Texas.
Bush has since said the United States 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.
