99942 Apophis (provisional designation 2004 MN₄) is a near-Earth asteroid and a potentially hazardous object, 450 metres (1,480 ft) by 170 metres (560 ft) in size. Observations eliminated the possibility of an impact on Earth in 2029, when it will pass the Earth at a distance of about 31,600 kilometres (19,600 mi) above the surface. It will also have a close encounter with the Moon, passing about 95,000 km from the lunar surface.

There was a brief period of concern in December 2004 when initial observations indicated a probability of 0.027 (2.7%) that the asteroid would hit Earth on Friday, April 13, 2029.

A small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole estimated to be 800 metres in diameter, which would have set up a future impact exactly seven years later on Easter Sunday, April 13, 2036. This possibility kept it at Level 1 on the 0 to 10 Torino impact hazard scale until August 2006, when the probability that Apophis would pass through the keyhole was determined to be very small and Apophis's rating on the Torino scale was lowered to Level 0. By 2008, the keyhole had been determined to be less than 1 km wide. During the short time when it had been of greatest concern, Apophis set the record for highest rating ever on the Torino scale, reaching Level 4 on December 27, 2004.

Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036 (probability less than one in a million). In February 2013 the estimated probability of an impact in 2036 was reduced to 7×10⁻⁹. It is now known that in 2036, Apophis will approach the Earth at a third the distance of the Sun in both March and December, about the distance of the planet Venus when it overtakes Earth every 1.6 years. Simulations in 2013 showed that the Yarkovsky effect might cause Apophis to hit a "keyhole" in 2029 so that it will come close to Earth in 2051, and then could hit another keyhole and hit Earth in 2068. But the chance of the Yarkovsky effect having exactly the right value for this was estimated as two in a million. Radar observations in March 2021 helped to refine the orbit, and in March 2021 the Jet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years. The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometres to now just a couple of kilometres, greatly enhancing predictions of future approaches. Entering March 2021, six asteroids each had a more notable cumulative Palermo scale rating than Apophis, and none of those has a Torino level above 0. However, Apophis will continue to be a threat possibly for thousands of years until it is removed from being a potentially hazardous object, for instance by passing close to Venus or Mars.

Apophis was discovered on June 19, 2004, by Roy A. Tucker, David J. Tholen, and Fabrizio Bernardi at the Kitt Peak National Observatory. On December 21, 2004, Apophis passed 0.0964 AU (14.42 Gm; 8.96 million mi) from Earth. Precovery observations from March 15, 2004, were identified on December 27, and an improved orbit solution was computed. Radar astrometry in January 2005 further refined its orbit solution. The discovery was notable in that it was at a very low solar elongation (56°) and at very long range (1.1 AU).^{[citation needed]}

When first discovered, the object received the provisional designation 2004 MN₄, and early news and scientific articles naturally referred to it by that name. Once its orbit was sufficiently well calculated, it received the permanent number 99942 (on June 24, 2005). Receiving a permanent number made it eligible for naming by its discoverers, and they chose the name "Apophis" on July 19, 2005. Apophis is the Greek name of Apep, an enemy of the Ancient Egyptian sun-god Ra. He is the Uncreator, an evil serpent that dwells in the eternal darkness of the Duat and tries to swallow Ra during his nightly passage. Apep is held at bay by Set, the Ancient Egyptian god of storms and the desert. Apophis will evolve from an Apollo- into an Aten-type asteroid as a result of the 2029 encounter. Apollo asteroids are usually named after Greek deities, Aten asteroids after Egyptian ones. The name Apophis is a nod to that: It is the Greek name of an Egyptian deity.

Tholen and Tucker, two of the co-discoverers of the asteroid, are reportedly fans of the television series Stargate SG-1. One of the show's persistent villains is an alien named Apophis. He is one of the principal threats to the existence of civilization on Earth through the first few seasons, and may have inspired the naming. Tholen denied reports that the asteroid was named after the TV character, stating that the connection is coincidental. In the fictional world of the show, the alien's backstory was that he had lived on Earth during ancient times and had posed as a god, thereby giving rise to the myth of the Egyptian god of the same name.

The mythological creature Apophis is pronounced with the accent on the first syllable (/ˈæpəfɪs/). In contrast, the asteroid's name is generally accented on the second syllable (/əˈpoʊfɪs/, or /əˈpɒfɪs/ as the name was pronounced in the TV series).