Sonic the Hedgehog (ソニック・ザ・ヘッジホッグ, Sonikku za Hejjihoggu) is a platform video game developed by Sonic Team and published by Sega for the Sega Genesis. Released in 1991, the game is the first installment in the Sonic the Hedgehog series, chronicling the adventures of the titular character in his quest to defeat the series' antagonist Doctor Ivo Robotnik. The game's story focuses on Sonic's efforts to stop Dr. Robotnik's plans for world domination, release the animals Dr. Robotnik has trapped, and collect six magical emeralds known as the Chaos Emeralds.

Development of Sonic the Hedgehog began in 1990. Sega ordered its AM-8 team to develop a game featuring a mascot for the company. The AM-8 team's name was changed to Sonic Team after a hedgehog was decided on as the main character.

Sonic the Hedgehog received positive reviews from critics, who praised the game's visuals. It was commercially successful, increasing the popularity of Sega's 16-bit console and establishing Sonic the Hedgehog as the company's mascot character. Its success led to the development of subsequent games in the Sonic the Hedgehog series, as well as the creation of a media franchise of spin-off products featuring the character.

Sonic the Hedgehog is a platform game in which the player controls the titular Sonic the Hedgehog. The goal of the game is to collect the six Chaos Emeralds, free the animals trapped inside robots and defeat Sonic's nemesis Doctor Ivo Robotnik ("Doctor Eggman" in the original Japanese release). The game is split up into zones, each of which is split into three acts. The player must navigate through each zone to progress. In the third act of each zone, the player confronts Dr. Robotnik in one of his vehicles. After completing the sixth zone, the player continues directly to the Final Zone for the last encounter with Robotnik. The player is given a certain amount of lives, which are lost when Sonic is attacked by an enemy or falls into a dangerous obstacle, crushed by a trap, drowns, falls into a pit, or spends over ten minutes in an act. If all lives are lost at any point in the game, the "Game Over" screen will appear, at which point the player can return to the beginning of the act with three lives if the player obtains continues from Special Stages.

The game plays as a 2D sidescrolling platformer. The gameplay centers around Sonic's ability to run at high speed through levels that include springs, slopes, high falls and loop-de-loops. The levels are populated with hazards in the form of animals Dr. Robotnik has trapped inside mechanical bodies (named "badniks" in the Western game manuals). The player must also avoid rows of sharp spikes, bottomless pits, and other obstacles. Sonic's main means of attack is the Spin Attack, where Sonic curls into a ball, which can be performed by jumping in the air or by pressing down on the D-Pad while moving on the ground. The move is used to destroy enemies and other obstacles in his way.

Scattered around each level map are gold rings, a signature item of the Sonic series. Collecting 100 rings rewards the player with an extra life. Rings act as a layer of protection against hazards—if Sonic is holding at least one ring when he collides with an enemy or dangerous obstacle, he will survive, but rings in his possession will be scattered. However, if he is hit without holding any rings, the player loses a life. Shields and temporary invincibility can also be collected to provide additional layers of protection.

Lamp posts that act as checkpoints allow Sonic to return to the point where the first post has been activated when he loses a life. If a checkpoint is activated and a life is lost as a result of running out of time, the time at the checkpoint will reset to 0:00.

When Sonic reaches the end of Act 1 or Act 2 of a zone with at least 50 rings, a large ring appears which the player can jump through to enter a Special Stage. In the Special Stages, Sonic is permanently curled up in his spinning animation, and bounces off the bumpers and walls of a 360° rotating maze. In these levels, the player may earn continues by collecting 50 rings; however, the main aim is to obtain the Chaos Emerald at the end of the maze, without colliding with any of the "goal blocks" that will instantly end the level. If Sonic collects every Chaos Emerald in the game, the player can view the good ending sequence.