The ICAO airport code (formally ICAO place indicator) is the four-letter airport designation code used to identify each airport worldwide.[1]​.

The codes are defined by the International Civil Aviation Organization (ICAO) and published in document Doc 7910/195, they are used for air traffic control and airline operations, such as flight planning. Unlike IATA codes that are used only to identify airports on airline, reservation and baggage boards, ICAO codes are also used to identify other locations such as weather stations or Area Control Centers.

Unlike IATA codes, ICAO codes have a regional structure (they are divided into "sections" of the world), are not duplicates, and are understandable. In general, the first letter is assigned by continent and represents a country or group of countries on that continent. The second letter usually represents a country within that region, and the remaining two are used to identify each airport. The exception to this rule is larger countries that have single-letter codes, where the other three letters identify the airport. In any case, ICAO codes provide geographic context unlike IATA codes. For example, if it is known that the ICAO code of Heathrow airport is EGLL, then it can be deduced that the airport with the code EGNH is somewhere in the United Kingdom (which is Blackpool International Airport). On the other hand, knowing that the IATA code of this airport is LHR, it is not possible to deduce the location of the airport with the PCI code with greater certainty (it is the airport William T. Piper Memorial Airport in Lock Haven, Pennsylvania, United States).

Currently, the letters I, J and X are not used as the first letter of any ICAO identifier. The letter Q is reserved for international radio communications and other special non-geographic uses (see Signal Q Code), the letter I is reserved for navigation aids, and J is for the planet Mars "Mars (planet)") based on the Jezero Crater "Jezero (crater)").

In the United States and Canada, most, but not all, airports have been assigned three-letter IATA codes that are the same as their ICAO code without the preceding letters or , such as the and codes (for Calgary International Airport) and the code pair and (for Washington Dulles International Airport, Chantilly, Virginia). These codes should not be confused with the "Callsign (ham radio)" callsigns used on radio or television stations, even when both countries use four-letter callsigns beginning with and . However, because the territories of Alaska, Hawaii and the United States have their own two-letter ICAO prefixes, the situation is similar to that of other smaller countries and the ICAO code of their airports is usually different from their corresponding three-letter FAA/IATA identifier.