Ex parte Merryman, 17 F. Cas. 144 (No. 9487), was a controversial U.S. federal court case that arose out of the American Civil War. It was a test of the authority of the President to suspend "the privilege of the writ of habeas corpus" under the Constitution's Suspension Clause, when Congress was in recess and therefore unavailable to do so itself. More generally, the case raised questions about the ability of the executive branch to decline to enforce judicial decisions when the executive believes them to be erroneous and harmful to its own legal powers.

John Merryman of Baltimore County, Maryland, was arrested in May 1861 and held prisoner in Fort McHenry in Baltimore and was the petitioner in the case "Ex parte Merryman" which was one of the best known habeas corpus cases of the American Civil War (1861–1865). Merryman was arrested for his involvement in the mob in Baltimore, specifically for his leadership in the destruction of telegraph lines, but was not charged, a right normally ensured by the writ of habeas corpus. The case was taken up by the federal circuit court and its current presiding judge who happened to be Chief Justice Roger B. Taney, a Democratic-leaning Marylander.