The Unconstitutionality of Slavery is an 1845 abolitionist essay written by the American abolitionist Lysander Spooner. In it, Spooner responds to Garrisonian abolitionists and proslavery theorists who argued that slavery was supported by the United States Constitution. Spooner claims that slavery is unconstitutional and cites natural law, colonial charters, and American founding documents to argue that there is no legal basis for the existence of slavery in the United States and that Congress is obligated to prohibit it.