Seward v. Comeau was a United States Supreme Court case that addressed the issue of whether a state could impose a tax on alcohol imported from another state. The case was brought by the state of New York, which had imposed a tax on alcohol imported from Canada. The defendant, Comeau, had purchased alcohol from Canada and brought it into New York, where he was charged with violating the state's tax law. The Supreme Court held that the state of New York could not impose a tax on alcohol imported from another state. The Court reasoned that the Commerce Clause of the United States Constitution prohibited states from imposing taxes on goods imported from other states. The Court noted that the purpose of the Commerce Clause was to ensure free trade between the states, and that any state law that interfered with this purpose was unconstitutional. The Court's decision in Seward v. Comeau was significant because it established the principle that states cannot impose taxes on goods imported from other states. This principle has been applied in numerous cases since then, and has been used to protect the free flow of goods between the states.

In Seward v. Comeau, the Supreme Court was asked to decide whether a New Brunswick law prohibiting the importation of liquor from other provinces violated the Commerce Clause of the United States Constitution. The majority opinion held that it did not violate this clause because it was within New Brunswick's police power to regulate its own internal commerce and protect public health and safety. However, Justice Field dissented on this point, arguing that while states have broad authority over their internal affairs, they do not have absolute control over them when such regulation affects interstate commerce. He argued that since alcohol is an article of trade between states and provinces in Canada, any restriction on its free flow would be unconstitutional under our federal system as it would interfere with Congress' exclusive right to regulate foreign or interstate commerce. Therefore he concluded that New Brunswick's prohibition against importing liquor from other provinces should be struck down as unconstitutional under the Commerce Clause.