Megatron is a fictional character and the main antagonist of the Transformers media franchise produced by the American toy company Hasbro and the Japanese toy company Takara Tomy. He is the tyrannical leader of the Decepticons, a villainous faction of alien robots that seeks to conquer their home planet of Cybertron and the rest of the known universe, and serves as the archenemy of Optimus Prime, the leader of the rival Autobot faction. As with all Cybertronians, Megatron can disguise himself by transforming into vehicles or weapons. His alternate modes have included a Walther P38 handgun, a particle-beam weapon, a telescopic laser cannon, a Cybertronian jet, and various tanks, depending on which continuity he is depicted in. His original designation, in some continuities, is D-16.

Megatron's most consistent origin portrays him as having risen up from being an oppressed worker to a gladiatorial champion who took the legendary name of one of the original Thirteen Primes—Megatronus—as his own. He shortened his name when he became a political revolutionary who attempted to reform Cybertron's corrupt governing body and called for an end to its decrepit caste system. As the mentor of the young Orion Pax, Megatron preached that freedom of self-determination was the right of all sentient beings. When Megatron grew corrupted by his power, Orion would utilize his teachings against him as Optimus Prime. In most incarnations, Megatron would eventually meet his demise at Optimus' hands, only to later be resurrected as Galvatron although some continuities have Galvatron as a separate entity from Megatron.

Megatron has become one of the franchise's most iconic characters and a widely recognized villain in popular culture. The character's popularity has seen him appear on a variety of merchandise, such as toys, clothing and collectible items, theme park attractions, and be referenced in a number of media. He has been adapted in live-action, animated, and video game incarnations, having been voiced by actors including Frank Welker, Corey Burton, Hugo Weaving, and Brian Tyree Henry.

The original Megatronus, better known as the Fallen, was one of the original Thirteen Primes created by Primus. Megatronus betrayed his creator by siding with Primus' dark twin, the malevolent planet-eater Unicron. In the final battle between Primus and Unicron, Megatronus fell victim to the same fate as his master, sucked through a black hole into another dimension. However, while Unicron emerged in another universe, the Fallen was not so fortunate, finding himself trapped in the "underspace" between dimensions.

The Fallen made his first appearance in the fifth volume of Transformers: The War Within. The character appears as the main antagonist in the second live action film, Transformers: Revenge of the Fallen, the 2009 sequel to the 2007 Transformers movie, voiced by Tony Todd. He has the ability to teleport and generate a shockwave upon reappearance. He wields a spear as his weapon and has the ability of telekinesis. Megatronus is also mentioned as one of Thirteen in Transformers: Exodus and later appeared as the main antagonist in the first-season finale of Transformers: Robots in Disguise, voiced by Gil Gerard.

Megatron is the founder of the Decepticon uprising and their most feared leader. Bob Budiansky, the writer for the Marvel Comics series, stated that originally Hasbro took issue with the name, saying it sounded too frightening. Budiansky responded that as the lead villain, that was the point. Hasbro later agreed with his reasoning, and approved the name "Megatron". Budiansky has said in interviews that it is a portmanteau of megaton and electronic.

Megatron is able to transform into a handgun, delivering more focused energy blasts than he could in his robotic form. He can adjust his size and mass as he transforms, to comfortably allow another Transformer or even a human being to wield him. His robot form has an arm-mounted fusion cannon. He can retract and replace his right hand with energy flail. He can fire electrical blasts from his hands, laser blasts from his eyes, and can reprogram computers with a port in his head.

Megatron is a Decepticon, one of the lineal descendants of the military hardware robots created by the Quintessons on their factory world of Cybertron. In a conflict between two robotic factions, the Decepticons and the Autobots, the Autobots' invention of transformation gave them an advantage, leading to their victory. The ensuing Golden Age of Cybertron was disrupted by the rise of Megatron, who led the Decepticons against the Autobots. Megatron's eventual defeat resulted in his rebirth as Galvatron through an alliance with the cosmic entity Unicron. Galvatron aimed to destroy the Autobot Matrix of Leadership, but his plans were thwarted, and he was thrown into space.