In re Coy is a United States Supreme Court case that addressed the issue of whether a state court could issue a writ of habeas corpus to a prisoner held in federal custody. The case involved a prisoner, Coy, who was held in federal custody in Arizona. Coy had been convicted of a crime in Arizona and was serving a sentence in a federal prison. Coy filed a petition for a writ of habeas corpus in the Arizona state court, arguing that his conviction was unconstitutional. The state court granted the writ and ordered Coy's release. The United States Supreme Court reversed the state court's decision, holding that the state court lacked jurisdiction to issue the writ of habeas corpus. The Court reasoned that the writ of habeas corpus was a federal remedy and that the state court lacked the authority to issue it. The Court further held that the state court's decision was in conflict with the Supremacy Clause of the United States Constitution, which provides that federal law is supreme over state law. In re Coy is an important case that established the principle that state courts cannot issue writs of habeas corpus to prisoners held in federal custody. The decision reaffirmed the principle that federal law is supreme over state law and that state courts cannot interfere with federal proceedings.

In In re Coy, the Supreme Court was asked to decide whether a Native American woman who had married an African-American man could be considered a citizen of the United States. The majority opinion held that she was not, as her marriage did not change her status from that of an Indian subject to tribal law and customs. Justice Field dissented, arguing that Congress had extended citizenship rights to all persons born in the United States regardless of race or ethnicity. He argued further that if Congress intended for this right only to apply to white citizens then it would have been explicitly stated in legislation rather than being implied through other laws which applied equally across racial lines. Furthermore, he noted that denying citizenship based on race violated both natural justice and fundamental principles of equality under the law enshrined in our Constitution.