There are several known species of extraterrestrial aliens in the universe, spanning 28 known galaxies. Clark Kent is himself an alien (Kryptonian), as are several of his friends and enemies. Six months after the event, referred to as "Contact", photographic evidence proving the existence of alien life on the planet Apokolips, adorns the internet and newspapers like the Daily Planet across the globe, instilling a mixed sense of concern, wonder, and fear amongst human population.

Humanoid/Near-Human Aliens

So far, all alien intelligences shown are humanoid or near-human in form, with most being outwardly indistinguishable from humans. All those seen so far seem to possess some form of superhuman abilities or powers, and are generally stronger and more durable than humans.

• Kryptonians are the most significant alien species that are close to humans and, in their natural environment on the planet Krypton, they are very similar to humans in abilities and appearance, but possess powers far beyond those of humans when exposed to the radiation of Earth's yellow sun. Prior to the destruction of their homeworld, Kryptonians achieved technology far more advanced than Earth's. The Kryptonian scientist Dax-Ur was able to survive for 100 years living on Earth, passing as a human, even with his powers suppressed by blue kryptonite; apparently, he was able to father a child with a human woman, suggesting the two species are able to interbreed. Clark Kent and his cousin Kara are the last known surviving Kryptonians, other than those in the Phantom Zone and the cloned Kandorian army.

• Almerac, a planet who's inhabitants are similar to humans in appearance, the most notable example being Maxima, the queen of the planet. Maxima displayed abilities similar to Kryptonians, and some not so much, like telepathy and an endorphin-stimulating kiss. She described her planet as having a similar surface and terrain as Earth.

• Winath, a planet described by Garth Ranzz, one of its inhabitants, as being like a "hunk of mud". Garth has the appearance of a human, and has the power of generating and manipulating electricity. It is not clear if his abilities are common to his species or if, like his comics counterpart, he acquired them somehow.
• Rokk Krinn and Imra Ardeen, like by Garth Ranzz members of Legion of Super-Heroes, but coming from diffferent planets, also have the appearance of a human, but special powers.
• Titan looks like a human in appearance, though his species remains unidentified. He was an escaped Phantom Zone prisoner who possessed super strength on par with Clark. he had a bone spike in his arm, but it isn't clear if this was part of his body or not. Although able to withstand the impact of Clark's punches, Titan died when he was impaled on his own spike.
• Gloria looks like a human in appearance, though her species remains unidentified. She was an escaped Phantom Zone prisoner who, In addition to having superhuman strength and durability, was able to implant seeds in human males. These seeds grew into thorny vines Gloria could animate and control. The vines were strong enough to hold Clark, and their thorns were able to pierce his body. Both the vines and Gloria were vulnerable to electricity.
• Aldar looks like a human in appearance, though his species remains unidentified. He was an escaped Phantom Zone prisoner. His only visible non-human features were his slitted red eyes. Aldar possessed strength, speed and durability sufficient to defeat Clark in a fight, and he ate human bone marrow. He was killed by the Martian Manhunter, who apparently burned a handprint into his back.

Phantom Wraiths

Phantom wraiths are aliens who were disembodied and imprisoned in the Phantom Zone. Their original forms were never seen.

• Baern became a phantom wraith after his was stripped of his physical body. Baern was a being made up of immense energy, who claimed to have destroyed his entire civilization alone. While possessing a human form, he was able to absorb vast quantities of nuclear radiation, project destructive energy blasts, and demonstrated enhanced strength, speed and durability in the Kryptonian range. Baern also interacted with and absorbed data and energy from Earth and Kryptonian technology.
• Dr. Hudson became a phantom wraith after his was stripped of his physical body. He was telepathic in nature, and could project mental illusions and pictures and could detect memories in detail of his host.
• The phantom who posed as Clark Kent, sometimes referred to as Bizarro, was apparently Kryptonian in origin; however, he was apparently created in a laboratory, making his true nature unclear. Unlike other phantoms, he burned out human hosts after only a day of possession; eventually, he created and possessed a genetically-modified Kryptonian body which was immune to this effect. This body absorbed energy differently than natural Kryptonians, being powered by kryptonite and weakened by sunlight. Bizarro's modified body was destroyed when it was overloaded with blue kryptonite radiation.

Shapeshifting Aliens

• Martians, the race of the Martian Manhunter. He possesses a number of Kryptonian-like abilities, such as flight and super strength, as well as additional powers, such as telepathy. The Martian Manhunter is able to rapidly regenerate from massive injury, but this power is inhibited by high levels of oxygen, forcing him to travel outside Earth's atmosphere to heal. In the online adventure Smallville Legends: Justice & Doom, he is shown shapeshifting; he recently displayed this ability in the main series in Checkmate. In Smallville, the only visible sign of his non-human status is his occasionally glowing red eyes, and his true Martian form was glimpsed when Doctor Fate exposed him to Mars' atmosphere in order to restore his powers. In the comics, the Martian Manhunter is the last survivor of his species; in Absolute Justice, he remarks that he is Mars' sole survivor and earlier stated that his family is dead and his civilization was destroyed.
• Brainiac is a unique artificial intelligence created on Krypton. It is composed of a fluid material that can re-shape itself to match the form, texture and color of either metal or human flesh. Brainiac can duplicate the abilities of Kryptonians, interact with both Earth and Kryptonian technology and computer systems, draw minerals from the human body, and extract information from a Kryptonian brain.
• Darkseid is an alien dark force that can transform into a gray cloud of smoke and ravens and seeks to corrupt humanity.

Non-Intelligent Alien Life-Forms

• A number of alien parasites were located in the walls of the Kawatche Caves; these parasites were capable of infecting humans with an inhibition-removing toxin, causing thrill-seeking behavior.