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Sonic the Hedgehog 2 is a 1992 platform game developed and published by Sega for the Sega Genesis. It follows Sonic as he attempts to stop Doctor Robotnik from stealing the Chaos Emeralds to power his space station, the Death Egg. Like the first Sonic the Hedgehog (1991), players traverse side-scrolling levels at high speeds while collecting rings and defeating enemies. Sonic 2 introduces Sonic's sidekick, Miles "Tails" Prower, controllable by a second player. It features faster gameplay and larger levels in comparison to the first game, in addition to a new multiplayer game mode and special stages featuring pseudo-3D graphics.

Although Sonic the Hedgehog had been a major success, Sega was slow to approve the development of a sequel. Sega Technical Institute (STI) founder Mark Cerny started working on Sonic 2 in November 1991, and members of the original development team—including programmer Yuji Naka and designer Hirokazu Yasuhara—moved to California to join the project. Sonic 2 was intended to be faster and more ambitious than the first game. The development suffered setbacks, including language and cultural differences between the Japanese and American staff, and numerous levels were cut due to time constraints and quality concerns. As with the first game, Dreams Come True bassist and songwriter Masato Nakamura composed the soundtrack.

As the sequel to one of the bestselling games of all time, Sonic 2 was widely anticipated, and Sega backed it with a $10 million marketing campaign. It was released in November 1992 to critical acclaim and became the second-bestselling Genesis game behind the original Sonic the Hedgehog, with more than 7 million copies sold by 1996. It was the fastest-selling and highest-grossing home entertainment product of 1992, grossing over $450 million by the end of the year. Critics generally considered Sonic 2 an improvement over the first game and praised the visuals, level design, gameplay, and music. The addition of Tails and the multiplayer mode received criticism, while the special stages divided opinion.

Sonic 2 played a major role in keeping Sega competitive during the console wars of the fourth generation of video game consoles in the early 1990s. It continues to receive acclaim, often cited as one of the greatest video games of all time and one of the best Sonic games. Sonic the Hedgehog 3 and Sonic & Knuckles followed in 1994. Sonic 2 has been rereleased on various platforms via compilations and emulation, and a remake for iOS and Android devices, developed using the Retro Engine, was released in December 2013.