Neptune, also called Poseidon, is a fictional character appearing in American comic books published by Marvel Comics. The character is based on the Roman God with the same name and his Greek counterpart. Neptune is the god of the sea in the Olympian pantheon, as well as patron god over Atlantis. Neptune first appeared in Tales to Astonish #70 and was adapted by Stan Lee and Gene Colan.

Neptune is the son of Cronus and Rhea, and was born in Mount Olympus. Zeus and Pluto are his brothers, and Hera and Vesta are his sisters. Neptune is responsible for the evolution of Homo mermanus and is still honored by the Atlanteans, unlike many Olympians who have little or no worshipers on Earth.

In modern times, Neptune sends Namor the Sub-Mariner on a quest for his trident in order to regain the Atlantean throne, appearing before him at various points. Neptune then appears to the Atlanteans, and declares Namor the true ruler of Atlantis.

Neptune abducts Namor from Atlantis to Hades at the behest of Zeus, as Zeus blames the Avengers for Hercules' bad physical condition. Neptune is forbidden to return to Earth by Zeus, despite his objections due to Atlantean worship.

Poseidon and the Olympians return to Earth following the death of Zeus. In a weakened state, he is abducted by Hippolyta and her Amazon warriors at the behest of Hera and Pluto. The Amazon princess Artume likewise forces Poseidon to tell her the location of the Omphalos. Poseidon is rescued from Artume by Hercules, Namor, and Namora.

Neptune sides with Athena when the other Olympian gods challenge her for leadership. Neptune chooses Namor as his proxy. The battle between the mortal proxies ends in a draw.

Neptune possesses the typical powers of an Olympian, including immense strength, stamina, durability, speed, and healing, as well as virtual immortality. Poseidon has the ability to wield vast cosmic/mystical energies for numerous purposes, such as inter-dimensional teleportation. As a sea god, Neptune is able to manipulate water at will, breathe oxygen/underwater, and communicate with all forms of sea life. He usually arms himself with a 6' 5" long enchanted trident, capable of divine magic.

Neptune appears in the "Namor" segment of The Marvel Super Heroes.^{[citation needed]}