Hudson & Smith v. Guestier was a case heard before the United States Supreme Court in 1812. The dispute arose when Hudson and Smith, two merchants from Philadelphia, sued Pierre Guestier for failing to pay them $4,000 plus interest that he had borrowed from them several years earlier. In his defense, Guestier argued that the debt should be discharged because of an act of Congress passed in 1797 which declared all debts contracted prior to June 1st of that year null and void if not paid within three years after its passage. The Supreme Court ultimately ruled against Guestier on the basis that this statute did not apply retroactively to debts already existing at the time it was enacted; therefore, since Hudson & Smith's loan predated 1797 they were entitled to payment by law regardless of any subsequent legislation passed by Congress.

In Hudson & Smith v. Guestier, the Supreme Court was asked to decide whether a contract between two parties that had been made in France should be enforced by an American court. The majority of the justices held that it should not because French law did not recognize such contracts as valid and enforceable. Justice Johnson dissented from this opinion, arguing that although French law may have invalidated the contract, there were still good reasons why an American court could choose to enforce it. He argued that if both parties had agreed to abide by its terms and conditions then they ought to be bound by them regardless of what foreign laws said about their validity or lack thereof. Furthermore, he noted that allowing courts in America to refuse enforcement on grounds of foreign laws would create uncertainty for those who wished to do business with foreigners since they would never know which country's laws applied at any given time.