Ex parte Juan Madrazzo was a case heard by the United States Supreme Court in 1833. The case involved an appeal from the Circuit Court of Louisiana, which had denied a writ of habeas corpus to Juan Madrazzo. At issue was whether or not Congress had authority under Article IV Section 2 Clause 1 of the Constitution (the “Fugitive Slave Clause”) to pass laws requiring state courts and magistrates to assist with apprehending runaway slaves. In its decision, the Supreme Court held that Congress did have such power and could require state officers to aid in enforcing federal fugitive slave laws. Furthermore, it found that states were obligated under their own constitutions as well as those of other states and territories within the Union to comply with these federal statutes when requested by another state's government. Finally, it concluded that denying Madrazzo his writ on this basis was proper given all applicable law at hand at time of trial.

In Ex Parte Juan Madrazzo, the Supreme Court held that a Mexican citizen who had resided in Texas since 1824 was not entitled to remain in the United States after Mexico ceded its territory to the U.S. The majority opinion found that because Madrazzo was an alien and did not become a naturalized citizen of either country before or after annexation, he could be removed from his residence without due process of law. Justice McLean dissented on this point, arguing that it would be unjust for someone who had lived peacefully and productively in one place for so long to suddenly have their rights taken away without any legal recourse. He argued further that if Congress intended such treatment they should have made it clear by passing laws specifically addressing these cases rather than relying on general principles of international law which were never meant to apply retroactively or deprive individuals of their rights as citizens under existing treaties between countries.