UFO: Enemy Unknown (original title), also known as X-COM: UFO Defense in North America, is a 1994 science fiction strategy video game developed by Mythos Games and MicroProse. It was published by MicroProse for DOS and Amiga computers, the Amiga CD32 console, and the PlayStation. Originally planned by Julian Gollop as a sequel to Mythos Games' 1988 Laser Squad, the game mixes real-time management simulation with turn-based tactics. The player takes the role of commander of X-COM – an international paramilitary and scientific organisation secretly defending Earth from an alien invasion. Through the game, the player is tasked with issuing orders to individual X-COM troops in a series of turn-based tactical missions. At strategic scale, the player directs the research and development of new technologies, builds and expands X-COM's bases, manages the organisation's finances and personnel, and monitors and responds to UFO activity.

Despite its troubled development, including having been almost cancelled twice, the game received strong reviews and was commercially successful, turning into a runaway sleeper hit and acquiring a cult following among strategy fans; several publications have listed Enemy Unknown as one of the best video games ever made, including IGN ranking it as the best PC game of all time in 2007. It was the first and best-received entry in the X-COM series and has directly inspired several similar games. An official remake of the game, XCOM: Enemy Unknown, was published in 2012, and subsequently regarded as also being one of the greatest games of all time.

The story of UFO: Enemy Unknown, set in the near-future at the time of the game's release, begins in the year 1998. The initial plot centres on increased reports of UFO sightings as tales of abductions and rumors of attacks by mysterious aliens become widespread. The nations of the world come to perceive this as a threat and attempt to form their own forces – such as Japan's Kiryu-Kai force – to deal with the crisis, but these efforts are unsuccessful. On 11 December, 1998, representatives from some of the most powerful nations in the world secretly meet in Geneva to discuss the issue. From this meeting is born the clandestine defence and research organisation Extraterrestrial Combat (X-COM), over which the player assumes control at the start of the game.

In the beginning, the player will only have access to conventional weapons, but as the game progresses, the player learns more about the enemy, their species, mutated creations, and technology. It is ultimately revealed that the "leaders" behind the alien invasion are a race known as Ethereals which possess powerful mind control abilities and enslave other races of aliens to perform their bidding and that their main base in the Solar System is located in Cydonia region of Mars. The player must then prepare the final assault team, attack Cydonia and destroy the mastermind behind the alien invasion, the biocomputer Alien Brain.

The game may end in several ways. If the player's performance is poor or worse for two consecutive months, the player runs a deep deficit for two consecutive months, all the player's bases are captured, or the player mounts an assault on the aliens' Mars base and loses, the game ends in defeat (in the PC version, the funding nations sign pacts with the aliens promising peace, but the aliens instead destroy every city and poison the water and air to destroy any resistance, the remaining survivors are put in slave camps to help reverse-terraform Earth for future alien colonisation; in the PlayStation version, the council of funding nations makes a futile attempt to negotiate with the aliens, who violently murder the negotiator). If, however, the player is victorious in the final attack, the game ends in mankind's victory.

The game takes place within two distinct views, called the Geoscape and the Battlescape. As described by GameSpy, UFO: Enemy Unknown "...melds an SSI Gold Box RPG with a highly detailed 4X game like Master of Orion, making it in some ways two entirely different games."

The game begins on 1 January, 1999, with the player choosing a location for their first base on the Geoscape screen: a global view representation of Earth as seen from space (displaying X-COM bases and aircraft, detected UFOs, alien bases, and sites of alien activity). The player can view the X-COM bases and make changes to them, equip fighter aircraft, order supplies and personnel (soldiers, scientists and engineers), direct research efforts, schedule manufacturing of advanced equipment, sell alien artefacts on black market to raise money, and deploy X-COM aircraft to either patrol designated locations, intercept UFOs, or send X-COM ground troops on missions using transport aircraft.

Simultaneously, the aliens may run various activities, from research missions designed to collect data about the Earth and its inhabitants, to searching for and attacking X-COM bases, to establishing their own bases, to terror attacks on cities with the aim of convincing governments to reduce X-COM's funding. The escalating alien activities need to be uncovered and countered, mostly by detecting and shooting down the alien crafts and sending ground troops to investigate and salvage the UFO landing and crash sites.