Sonic Adventure (ソニックアドベンチャー, Sonikku Adobenchā) is a platform video game developed by Sonic Team and published by Sega. It was originally released for the Dreamcast as a launch title in Japan in 1998, and worldwide in 1999. It was the first game in the main Sonic the Hedgehog series since Sonic & Knuckles was released in 1994.

After series villain Doctor Eggman releases an ancient monster, protagonist Sonic the Hedgehog vows to stop him collecting the Chaos Emeralds. The other playable characters are flying fox Miles "Tails" Prower, love interest Amy Rose, Knuckles the Echidna, who plans to reassemble the Master Emerald, angler Big the Cat, and E-102 Gamma, one of Eggman's robots.

Sonic Adventure received generally positive reviews for its graphics, gameplay, and soundtrack. It became the best-selling Dreamcast game. A sequel, Sonic Adventure 2, was released in 2001 for the Dreamcast. Enhanced ports of the game were released in 2003 for the Nintendo GameCube and Microsoft Windows. It has since been re-released as a downloadable game for Xbox 360 via Xbox Live Arcade, and PlayStation 3 via PlayStation Network in 2010. It is also included in the Dreamcast Collection for the Xbox 360.

Sonic Adventure 2 (ソニックアドベンチャー2(ツー), Sonikku Adobenchā Tsū) is a 2001 video game developed by Sonic Team USA and published by Sega for the Dreamcast as part of the Sonic the Hedgehog series. It was released as the last Sonic game for a Sega console after the company decided to leave the console hardware business. A port for the Nintendo GameCube, Sonic Adventure 2 Battle, was first released in Japan in December 2001, with new content. The game was later released digitally for the PlayStation 3, Xbox 360, and Microsoft Windows in 2012.

The sequel to Sonic Adventure, Adventure 2 features two good-vs.-evil stories: a hero story of Sonic the Hedgehog, Miles "Tails" Prower and Knuckles the Echidna as they attempt to save the world, and a dark story following Shadow the Hedgehog, Doctor Eggman and Rouge the Bat in their attempt to conquer it. The stories are divided into three gameplay styles: traditional, fast-paced platforming for Sonic and Shadow; multi-directional shooting for Tails and Eggman, and action-exploration for Knuckles and Rouge. The game also includes an extensive Chao-raising system.