The Dormant Commerce Clause, or Negative Commerce Clause, in American constitutional law, is a legal doctrine that courts in the United States have inferred from the Commerce Clause in Article I of the US Constitution. The primary focus of the doctrine is barring state protectionism. The Dormant Commerce Clause is used to prohibit state legislation that discriminates against, or unduly burdens, interstate or international commerce. Courts first determine whether a state regulation discriminates on its face against interstate commerce or whether it has the purpose or effect of discriminating against interstate commerce. If the statute is discriminatory, the state has the burden to justify both the local benefits flowing from the statute and to show the state has no other means of advancing the legitimate local purpose.

For example, it is lawful for Michigan to require food labels that specifically identify certain animal parts, if they are present in the product, because the state law applies to food produced in Michigan as well as food imported from other states and foreign countries; the state law would violate the Commerce Clause if it applied only to imported food or if it was otherwise found to favor domestic over imported products. Likewise, California law requires milk sold to contain a certain percentage of milk solids that federal law does not require, which is allowed under the Dormant Commerce Clause doctrine because California's stricter requirements apply equally to California-produced milk and imported milk and so does not discriminate against or inappropriately burden interstate commerce.

The doctrine was initially envisioned by Chief Justice John Marshall in the 1820s.

The idea that regulation of interstate commerce may to some extent be an exclusive Federal power was discussed even before adoption of the Constitution. On September 15, 1787, the Framers of the Constitution debated in Philadelphia whether to guarantee states the ability to lay duties of tonnage without Congressional interference so that the states could finance the clearing of harbors and the building of lighthouses. James Madison believed that the mere existence of the Commerce Clause would bar states from imposing any duty of tonnage: "[Madison] was more and more convinced that the regulation of Commerce was in its nature indivisible and ought to be wholly under one authority." Roger Sherman disagreed: "The power of the United States to regulate trade being supreme can control interferences of the State regulations when such interferences happen; so that there is no danger to be apprehended from a concurrent jurisdiction." Sherman saw the commerce power as similar to the tax power, the latter being one of the concurrent powers shared by the federal and state governments. Ultimately, the Philadelphia Convention decided upon the present language about duties of tonnage in Article I, Section 10, which says: "No state shall, without the consent of Congress, lay any duty of tonnage ..."

The word "dormant", in connection with the Commerce Clause, originated in dicta of Chief Justice John Marshall. For example, in the case of Gibbons v. Ogden, Marshall wrote that the power to regulate interstate commerce "can never be exercised by the people themselves, but must be placed in the hands of agents, or lie dormant." In concurrence, Justice William Johnson was even more emphatic that the Constitution is "altogether in favor of the exclusive grants to Congress of power over commerce." Later, in the case of Willson v. Black-Bird Creek Marsh Co., Marshall wrote: "We do not think that the [state] act empowering the Black Bird Creek Marsh Company to place a dam across the creek, can, under all the circumstances of the case, be considered as repugnant to the power to regulate commerce in its dormant state, or as being in conflict with any law passed on the subject."

If Marshall was suggesting that the power over interstate commerce is an exclusive federal power, the Dormant Commerce Clause doctrine eventually developed very differently: it treats regulation that does not discriminate against or unduly burden interstate commerce as a concurrent power, rather than an exclusive federal power, and it treats regulation that does so as an exclusive federal power. Thus, the modern doctrine says that congressional power over interstate commerce is somewhat exclusive but "not absolutely exclusive". The approach began in the 1851 case of Cooley v. Board of Wardens, in which Justice Benjamin R. Curtis wrote for the Court: "Either absolutely to affirm, or deny that the nature of this [commerce] power requires exclusive legislation by Congress, is to lose sight of the nature of the subjects of this power, and to assert concerning all of them, what is really applicable but to a part." The first clear holding of the Supreme Court striking down a state law under the Dormant Commerce Clause came in the 1873 case Reading Railroad Company v. Pennsylvania.

Justice Anthony Kennedy has written that: "The central rationale for the rule against discrimination is to prohibit state or municipal laws whose object is local economic protectionism, laws that would excite those jealousies and retaliatory measures the Constitution was designed to prevent." In order to determine whether a law violates a so-called "dormant" aspect of the Commerce Clause, the court first asks whether it discriminates on its face against interstate commerce. In this context, "discrimination" simply means differential treatment of in-state and out-of-state economic interests that benefits the former and burdens the latter.

Thus, in a dormant Commerce Clause case, a court is initially concerned with whether the law facially discriminates against out-of-state actors or has the effect of favoring in-state economic interests over out-of-state interests. Discriminatory laws motivated by "simple economic protectionism" are subject to a "virtually per se rule of invalidity", which can only be overcome by a showing that the State has no other means to advance a legitimate local purpose.