District of Columbia v. Gallaher was a United States Supreme Court case that addressed the issue of whether a state could impose a tax on a non-resident's income from real estate located within the state. The case was brought by the District of Columbia against William Gallaher, a non-resident of the District. The District argued that it had the right to impose a tax on Gallaher's income from real estate located within the District. The Supreme Court held that the District did not have the authority to impose a tax on Gallaher's income from real estate located within the District. The Court reasoned that the District was not a state, and therefore did not have the power to impose a tax on a non-resident's income from real estate located within the District. The Court further held that the District was not authorized to impose a tax on a non-resident's income from real estate located within the District under the Constitution. The Court's decision in District of Columbia v. Gallaher established that a state cannot impose a tax on a non-resident's income from real estate located within the state. This decision has been cited in numerous subsequent cases, and has been used to support the principle that a state cannot impose a tax on a non-resident's income from real estate located within the state.

In District of Columbia v. Gallaher, the Supreme Court was asked to decide whether a statute passed by Congress that allowed for the collection of taxes on real estate in Washington D.C., which had been previously exempted from taxation, violated the Fifth Amendment's Takings Clause. The majority opinion held that it did not violate this clause because there was no taking without just compensation and thus no violation of due process rights under the Constitution. However, Justice Field dissented from this decision and argued that while Congress may have had authority to pass such a law, it still constituted an unconstitutional taking without just compensation as required by the Fifth Amendment’s Takings Clause since property owners were being deprived of their right to use or dispose of their land free from government interference or control without any form of payment or other consideration given in return. He further noted that if such laws are allowed then all private property would be subject to taxation at any time with little recourse for those affected by them since they could not challenge these laws until after they were already enforced against them and thus suffer irreparable harm before having an opportunity to seek redress through legal action.