United States v. Ramsay was a United States Supreme Court case that addressed the issue of whether a foreign-born person who had been naturalized in the United States was still subject to the jurisdiction of the country of his birth. The case involved John Ramsay, a native of Scotland who had been naturalized in the United States in 1868. In 1876, he was arrested in Scotland on a charge of larceny and was tried and convicted in a Scottish court. The United States government argued that Ramsay was still subject to the jurisdiction of the United States and that the Scottish court had no authority to try him. The Supreme Court agreed, ruling that Ramsay was still subject to the jurisdiction of the United States and that the Scottish court had no authority to try him. The Court held that a foreign-born person who had been naturalized in the United States was still subject to the jurisdiction of the United States and that the laws of the United States applied to him. The Court also held that the Scottish court had no authority to try him because he was a citizen of the United States.

In United States v. Ramsay, the Supreme Court was asked to decide whether a federal court had jurisdiction over a case involving an alleged violation of customs laws by two individuals who were not citizens of the United States. The majority opinion held that such cases did fall within the purview of federal courts and thus affirmed the lower court's decision in favor of the government. Justice Field dissented from this ruling, arguing that Congress had never intended for non-citizens to be subject to prosecution under U.S. customs laws and therefore it would be unconstitutional for them to do so without explicit authorization from Congress itself or some other source with similar authority. He further argued that allowing such prosecutions could lead to potential abuses against foreign nationals who may not understand their rights under U.S law or have access to legal counsel if they are accused of violating any applicable regulations or statutes governing imports into America