Simonston v. Sibley is a United States Supreme Court case that was decided in 1887. The case involved a dispute between two parties over the ownership of a piece of land in the state of Georgia. The plaintiff, Simonston, claimed that he had purchased the land from the defendant, Sibley, in 1871. Sibley, however, argued that he had never sold the land to Simonston and that he was still the rightful owner. The Supreme Court ultimately sided with Simonston, ruling that he had acquired the land through a valid contract with Sibley. The Court held that the contract was binding and that Sibley had no right to reclaim the land. The Court also noted that the contract had been executed in good faith and that Simonston had acted in reliance on the contract. The Court's decision in Simonston v. Sibley established the principle that a contract is binding and enforceable even if it is not in writing. This ruling has been cited in numerous subsequent cases and has become an important part of contract law in the United States.

In Simonton v. Sibley, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving land located in two different states. The majority opinion held that the state court did not have jurisdiction because it lacked authority to adjudicate matters concerning property situated in another state. However, Justice Field dissented from this decision and argued that the Constitution does not prohibit such an exercise of power by a State court when both parties are citizens of that same State. He further noted that if Congress has granted permission for such action then there is no reason why States should be prohibited from exercising their own powers as well. Furthermore, he argued that allowing one party to bring suit against another citizen of its own State would promote justice and prevent fraud or injustice between individuals living within the same boundaries without having to resort to federal courts which may be too distant or expensive for some litigants.