Phillips v. United States was a United States Supreme Court case in which the Court held that the federal government had the power to tax the income of individuals. The case arose when the petitioner, William Phillips, refused to pay a federal income tax on the grounds that it was unconstitutional. The Court held that the federal government had the power to tax the income of individuals under the Sixteenth Amendment to the United States Constitution. The Court also held that the tax was not a direct tax, and thus did not violate the Constitution. The Court's decision established the federal government's power to tax the income of individuals, and has been cited in numerous subsequent cases.

In the case of Phillips v. Commissioner, Justice Harlan delivered a dissenting opinion in which he argued that the Court should have found for the petitioner on constitutional grounds. He argued that Congress had no authority to impose an income tax on individuals and corporations without apportionment among states according to population as required by Article I, Section 9 of the Constitution. Furthermore, he noted that while Congress was granted certain powers under Article I, it did not possess any inherent power to levy taxes or pass laws regulating commerce between states; such powers were reserved exclusively for state governments. Therefore, since there was no evidence presented at trial indicating otherwise, Justice Harlan concluded that Congress had exceeded its authority when it imposed an income tax without apportionment among states according to population and thus violated both Articles I and IX of the Constitution.