The United States district courts are the trial courts of the U.S. federal judiciary. There is one district court for each federal judicial district. Each district covers one U.S. state or a portion of a state. There is at least one federal courthouse in each district, and many districts have more than one. District court decisions are appealed to the U.S. court of appeals for the circuit in which they reside, except for certain specialized cases that are appealed to the U.S. Court of Appeals for the Federal Circuit or directly to the U.S. Supreme Court.

District courts are courts of law, equity, and admiralty, and can hear both civil and criminal cases. But unlike U.S. state courts, federal district courts are courts of limited jurisdiction, and can only hear cases that involve disputes between residents of different states, questions of federal law, or federal crimes.

Unlike the U.S. Supreme Court, which was expressly established by Article III of the Constitution, the district courts were established by Congress pursuant to authority delegated by Article III through the enacting of a federal statute, the Judiciary Act of 1789. There is no constitutional requirement that district courts exist at all.

During the drafting and ratification of the Constitution, some opponents of a strong federal judiciary argued that the federal courts ought to be limited to the Supreme Court, which would hear appeals only from state courts. In other words, the state courts would be treated as federal tribunals under Article I of the Constitution for the purpose of hearing disputes under federal law, but their judges would not become officers of the federal government. Edward Carrington advocated this position in a letter to James Madison, and it was also discussed by Alexander Hamilton in Federalist No. 81. However, this view did not prevail, and the first Congress created the district court system that is still in place today. Pursuant to the Constitution, nonetheless, state courts retain the power of concurrent jurisdiction in most federal matters.

When the Act was first passed, there were thirteen districts created among the eleven states which had ratified the Constitution by that point. When North Carolina and Rhode Island voted to ratify, a district was created for each of them, bringing the number of districts to fifteen.

The territories (insular areas) of Guam, the Northern Mariana Islands, and the United States Virgin Islands each have one territorial court; these courts are called "district courts" and exercise the same jurisdiction as district courts, but differ from district courts in that territorial courts are Article IV courts, with judges who serve ten-year terms rather than the lifetime tenure of judges of Article III courts, such as the district court judges.

American Samoa having neither a district court nor a federal territorial court, its federal cases are sent either to the District of Columbia or Hawaii. The Philippines, previously part of the United States, were never part of the U.S. federal court system.

There are 89 districts in the 50 states, with a total of 94 districts including territories. There is at least one judicial district for each state, the District of Columbia, and Puerto Rico.