The planet Neptune has been used as a reference and setting in various films and works of fiction:
In H. G. Wells's short story The Star, Neptune is destroyed in a collision with another supermassive object which reduces its orbital velocity to zero; the wreckage falls into the Sun, narrowly missing Earth.
In the Captain Future series, Neptune is portrayed as a sea planet, not out of any scientific theory but evidently because Neptune is the Roman sea god.
In Olaf Stapledon's 1930 epic novel Last and First Men, Neptune is the final home of the highly evolved human race. The planet is depicted as having a dense atmosphere but with a solid surface.
In Hugh Walters' 1968 novel Nearly Neptune, the first manned expedition to Neptune ends in apparent disaster as a fire destroys vital equipment on board the spacecraft as it nears the planet.
The planet served as the backdrop for the 1997 science fiction/horror film Event Horizon.
The pilot of the TV movie Virtuality centers around a starship preparing to make a flyby of Neptune before leaving the solar system.
In 2001 (or 2151, depending on your point of view), Star Trek made an official canon reference to the planet. Captain Jonathan Archer and his chief engineer were taking a final inspection pass around their new ship, the NX-01 Enterprise. Of the vessel's anticipated best speed, and with awe in his voice, Archer said, "Neptune and back in six minutes."
Mothstorm (2008), a book in the Larklight Trilogy by Philip Reeve. Neptune is called Hades. The lizard-like Silth tow their miniature Sun into orbit of it, allowing them to inhabit it and rename it Snil. The remainder of the giant moths they farmed are taken there also.
In the animated TV series Futurama (1999 - 2003, 2007 - 2009), Neptunians are a purple-skinned, four-armed race of humanoids that coexist peacefully with humans across the solar system. Elzar the cook, is a Neptunian. Neptune itself is only seen in "A Tale of Two Santas" and in "Bender's Big Score," where its North Pole is the location of Robot Santa's Death Fortress, with stunted Neptunian "elves" working for him, who are treated badly. Yetis are shown in Bender's Big Score. The planet is depicted as icy, but again, since only its north pole is shown, this may not be indicative of the entire planet.
In the Japanese AnimeUrusei Yatsura, Neptune is an icy, cold place which is the home of Oyuki, one of Lum's childhood alien friends.
In Space Patrol (1962) - episode The Slaves of Neptune, the crew of the Galasphere are sent to solve the mystery of a spaceship sending colonists to Pluto which disappeared near Neptune. On approach to Neptune Dart, Slim and Husky fall under the hypnotic influence of Neptunian overlord Tyro who is using his powers to trap Earth colonists as slaves.
In Grant Morrison's DC One Million (1998), all the planets of the solar system are overseen by one member of the future descendants of the Justice League. Neptune is overseen by the Aquaman of the 853rd century, and is described as being covered in oceans.
In 1975, the Mego Corporation created an eight-inch "Neptunian"  action figure doll for its first line of "Star Trek Aliens." Although elaborately designed as a monstrous reptilian with a long, thin head, a scaly green-and-red jumpsuit (with wings sewn under each arm), and removable, reptilian, plastic green gloves and boots, the Neptunian is particularly noteworthy because it never appeared in any Star Trek TV episode or movie, before or since.
In All-Star Comics #13 the Justice Society of America are gassed by Nazis and sent to different planets. Dr. Mid-Nite lands on Neptune, an ice-covered planet with some sort of a man-built communications system on the surface. He is met by several creatures and taken to meet Hydara, ruler of the subterranean people and made a slave. Just then, a person runs in and states that he has caught the dreaded plague that has been killing off his people. Dr. Mid-Nite deduces that they merely have the measles, which he can easily cure. He asks for herbs and makes a solution that the people can absorb through their feet-roots. As a reward, he is given a complete set of books describing their secrets of plant growth and surgical work.
In Marvel Family #27 Neptune is shown to be inhabited by a race of robots that use 'sounds' for currency. Cap meets a Neptunian who wants to capture a human for his freak show. Radio waves from Billy Batson's show are picked up by the ship and a device is activated that pulls him towards the ship. Billy summons Captain Marvel and easily defeats the Neptunians. He then foolishly turns back, thinking the Neptunians won't be scared of him. However he is hit on the head, and knocked out by the robot. The unconscious Billy is securely tied up and gagged. He finds himself in a cage with a cover on him. He gets the loose end of his gag on the cover and when it is pulled away his gag is pulled of. He transforms and the Neptunians think he is not human, but a robot due to his invulnerability. He flies the owner away from the mob and tells him not to exhibit people because they look different. However the robot then finds a Chink-Chink, a torotise-like creature that makes that sound and takes it back with him, hoping to exhibit it.
In Marvel Family #36 all life on Neptune is wiped out by the Invaders from Infinity (see List of Captain Marvel (DC Comics) enemies), who want to systematically destroy all forms of matter so they have more space to move in. Although never actually seen, it appears an outpost on Pluto is manned by Neptunians, who appear humanoid. The Neptunians are advanced enough to contact Earth. When the Marvels go to Neptune, they find holes, and think the Neptunians must have been a mole-like race. The Neptunians are avenged when the Invaders are imprisoned and destroyed.
In July 1958 Superman journeys to Neptune to obtain a stone Superman head once sculpted by the Neptunians in his honor. The sculpture is one of a series of so-called "space trophies" which the Man of Steel gathers for inclusion in a time capsule which the Metropolis Museum plans to bury in the ground as a gift for the people of the fiftieth century A.D. (S No. 122/1: "The Secret of the Space Souvenirs").
In Marvel Family #16 it is revealed the mightiest being on Neptune is an octopus-like creature. He is kidnapped by a Plutonian tyrant to fight the mightiest beings of other planets, but is beaten by Captain Marvel.
The Gobsmacking Galaxy, an entry in the children's non-fiction series The Knowledge written by Kjartan Poskitt, humorously describes hypothetical alien life forms which might evolve on planets in the solar system; the extreme cold of Neptune precludes any actual physical body, so the Neptunian creature is a being of pure electromagnetic energy, gamma rays and photons, while the Triton creature would be the same, except wearing an anorak to cope with the slightly lower temperature.
Three levels of Descent take place on Neptune or its moons. Level 22 is set on Neptune in a storage depot. The setting for level 23 was also a storage depot, this time on Neptune's largest moon, Triton. Level 24 took place on Nereid, in a volatile materials mine.
Triton was used as a temporary base of operations for the so-called 'Earthguard' by the Spathi in the computer game Star Control II.
In Red Dwarf: Infinity Welcomes Careful Drivers one of the original ill-fated crew members revealed he had bought a house on Triton but would have to wear a spacesuit in his house as an oxygen atmosphere "had not been installed yet".
"Neptune's moon" (only Triton had been discovered at the time of writing) had a Boskonian base on it in E. E. Smith's Lensman series, which was destroyed by the Galactic Patrol only a couple of pages after its existence was first mentioned.