Aircraft hijacking (also known as airplane hijacking, skyjacking, plane hijacking, plane jacking, air robbery, air piracy, or aircraft piracy, with the last term used within the special aircraft jurisdiction of the United States) is the unlawful seizure of an aircraft by an individual or a group. Dating from the earliest of hijackings, most cases involve the pilot being forced to fly according to the hijacker's demands. There have also been incidents where the hijackers have overpowered the flight crew, made unauthorized entry into the cockpit and flown them into buildings—most notably in the September 11 attacks—and in some cases, planes have been hijacked by the official pilot or co-pilot, such as with Ethiopian Airlines Flight 702.

Unlike carjacking or sea piracy, an aircraft hijacking is not usually committed for robbery or theft. Individuals driven by personal gain often divert planes to destinations where they are not planning to go themselves. Some hijackers intend to use passengers or crew as hostages, either for monetary ransom or for some political or administrative concession by authorities. Various motives have driven such occurrences, such as demanding the release of certain high-profile individuals or for the right of political asylum (notably Ethiopian Airlines Flight 961), but sometimes a hijacking may have been affected by a failed private life or financial distress, as in the case of Aarno Lamminparras in Finnair Flight 405. Hijackings involving hostages have produced violent confrontations between hijackers and the authorities, during negotiation and settlement. In several cases – most famously Air France Flight 139, Lufthansa Flight 181, and Air France Flight 8969 – the hijackers were not satisfied and showed no inclination to surrender, resulting in the deployment of counterterrorist police tactical units or special forces to rescue the passengers.

In most jurisdictions of the world, aircraft hijacking is punishable by life imprisonment or a long prison sentence. In most jurisdictions where the death penalty is a legal punishment, aircraft hijacking is a capital crime, including in China, India, Liberia, and the U.S. states of Georgia and Mississippi.

Airplane hijackings have occurred since the early days of flight. These can be classified in the following eras: Pre-1929, 1929–1957, 1958–1979, 1980–2000, and 2001–present. Early incidents involved light planes, but this later involved passenger aircraft as commercial aviation became widespread.

One of the first accounts of an aircraft hijacking dates to 1919, sometime during the short existence of the Hungarian Soviet Republic (21 March – 1 August). Hungarian polymath Baron Franz Nopcsa von Felső-Szilvás was a spy for Austria-Hungary in World War I, whose defeat and dissolution ceded Nopcsa's native Transylvania to Romania. Under the new socialist state government, Nopcsa and Bajazid Elmaz Doda (his secretary and boyfriend) were unable to obtain a passport to leave the country. To circumvent this, Nopcsa forged documents from the Ministry of War that convinced the military commander at the Mátyásföld Airport [fr; hu] on the outskirts of Budapest to provide him and Doda with a small airplane and a pilot. Somewhere over Győr, approximately halfway between Budapest and their supposed destination of Sopron, Franz pulled out a revolver, held it to the pilot's head, and demanded to be flown to Vienna (then a part of the also transitionary Republic of German-Austria (12 November 1918 – 10 September 1919)). Nopcsa and Doda successfully arrived in Vienna, where they lived until Nopcsa's murder-suicide of Doda in 1933 after years of depression and financial destitution.

Between 1929 and 1957, there were fewer than 20 incidents of reported hijackings worldwide; several occurred in Eastern Europe.

An early but unconfirmed account of hijacking occurred in December 1929. J. Howard "Doc" DeCelles was flying a postal route for a Mexican firm, Transportes Aeras Transcontinentales, ferrying mail from San Luis Potosí to Torreon and then on to Guadalajara. Saturnino Cedillo, the governor of the state of San Luis Potosí, ordered him to divert. Several other men were also involved, and through an interpreter, DeCelles had no choice but to comply. He was allegedly held captive for several hours under armed guard before being released.

The first recorded aircraft hijack took place on February 21, 1931, in Arequipa, Peru. Byron Richards, flying a Ford Tri-Motor, was approached on the ground by armed revolutionaries. He refused to fly them anywhere during a 10-day standoff. Richards was informed that the revolution was successful and he could be freed in return for flying one of the men to Lima.