The International Court of Justice at The Hague has ruled that the US violated the rights of 51 Mexicans on death row in American prisons.
Mexico wants new trials for the men
The court found that the prisoners did not receive a fair trial because they were not told of their rights to consular assistance.
Mexican lawyers argued that consular help could have made the difference between life and death.
Presiding judge Shi Jiuyong said the men's convictions should be reviewed.
"The US should provide by means of its own choosing
meaningful review of the conviction and sentence" in all but one of the 52 cases presented, he said.
Arturo Dajer, legal adviser to Mexico's Department of Foreign Relations, called the ruling "a triumph of international law".
The 1963 Vienna Convention, which both countries have signed, required the men to be informed of their rights to consular aid.
US lawyers have dismissed the case as a publicity stunt, claiming the men have already had fair trials.
The court ruled last year that three of the Mexican nationals on death row should be granted a stay of execution until it issued a judgement.
Washington argued that ruling infringed America's sovereignty over its criminal justice system.
The court's decisions are legally binding.
But in a similar case, the court ordered the US to stay the execution of a German national in 1999 - he was put to death nevertheless.
The current case has put pressure on Mexico to reform its own justice system.
Hours before the judgement, Mexico's President Vicente Fox announced a reform package that includes plans to substitute the death penalty, which exists for military personnel, with 30- to 60-year prison sentences.
The death penalty has not been applied in Mexico's army
since 1961 and is theoretically reserved for the worst
offences, such as treason or serious dereliction, the Associated Press reported.