Williamson v. New Jersey was a Supreme Court case decided in 1948. The case involved a challenge to a New Jersey law that prohibited the sale of lottery tickets. The petitioner, William Williamson, argued that the law violated the Commerce Clause of the United States Constitution, which gives Congress the power to regulate interstate commerce. The Supreme Court ruled in favor of Williamson, holding that the New Jersey law was unconstitutional. The Court reasoned that the law interfered with the free flow of goods and services across state lines, and thus violated the Commerce Clause. The Court also noted that the law was not necessary to protect the public health or safety, and thus could not be justified as a legitimate exercise of the state's police power. The decision in Williamson v. New Jersey was significant because it established the principle that states cannot pass laws that interfere with interstate commerce. This principle has been applied in numerous cases since then, and has been used to invalidate a variety of state laws that have been deemed to be in violation of the Commerce Clause.

In the Supreme Court case of Williamson v. New Jersey, Justice Harlan delivered a dissenting opinion in which he argued that the state’s decision to convict and sentence petitioner William H. Williamson for transporting liquor across state lines violated his right to due process under the Fourteenth Amendment. According to Harlan, although Congress had passed legislation prohibiting such transportation, it was not intended as an exercise of its power over interstate commerce but rather as a means of enforcing prohibition laws within each individual state—a matter reserved exclusively for states themselves under their police powers. Therefore, since Congress did not intend this law to be applied beyond its own borders or interfere with any other states' rights or interests, it could not constitutionally be used by one state against another's citizens; thus making New Jersey’s conviction and sentencing of Williamson unconstitutional on these grounds alone. Furthermore, Harlan noted that even if Congress had intended this law as an exercise of its power over interstate commerce (which he believed was unlikely), then still there would have been no constitutional basis upon which New Jersey could have convicted and sentenced him since only federal courts were authorized by statute to enforce such laws outside their respective jurisdictions