Havemeyer v. Iowa County was a case heard by the United States Supreme Court in 1865. The plaintiff, Havemeyer, owned land that had been taxed for several years prior to his purchase of it and he argued that this taxation violated the Constitution's prohibition against ex post facto laws. The defendant county contended that since they were unaware of Havemeyer's ownership at the time of taxation, their actions did not constitute an ex post facto law violation. In its ruling, the court held in favor of Havemeyer and found that even though there may have been no intent on behalf of Iowa County to violate any constitutional provisions when levying taxes on property without knowledge as to who owned it at the time; such action still constituted an unconstitutional act because it imposed a burden upon someone after-the-fact (ex post facto). Therefore, all taxes collected from Havemeyer while he was owner must be refunded with interest accrued during collection period.

In Havemeyer v. Iowa County, the Supreme Court was asked to decide whether a state statute that imposed taxes on certain railroad companies violated the Constitution's Contract Clause. The majority opinion held that it did not violate the clause because there was no contract between the state and those particular railroads which would be impaired by such taxation. However, in dissent Justice Field argued that while there may have been no express contract between these railroads and the state of Iowa, an implied contract existed due to prior legislation passed by Congress granting land grants for construction of these railroads with conditions attached regarding taxation. Thus he concluded that this tax law violated both federal and constitutional law as it impaired an existing contractual obligation between them.