Sonic Adventure is a 1998 platform game developed by Sonic Team and published by Sega for the Dreamcast. It was the first main Sonic the Hedgehog game to feature 3D gameplay. It follows Sonic the Hedgehog, Miles "Tails" Prower, Knuckles the Echidna, Amy Rose, Big the Cat, and E-102 Gamma in their quests to collect the Chaos Emeralds and stop Doctor Ivo Robotnik from unleashing Chaos, an ancient evil. Controlling one of the six characters—each with their own abilities—players complete levels to progress the story. Sonic Adventure retains many elements from prior Sonic games, such as power-ups and the ring-based health system. Players can play minigames such as racing and interact with Chao, a virtual pet.

Sonic Team began developing Sonic Adventure in 1997, after the cancellation of the Sega Saturn game Sonic X-treme. Led by director Takashi Iizuka and producer Yuji Naka, the team strove to reinvent Sonic for the 3D era of video games. Adventure features a stronger emphasis on storytelling and role-playing elements in contrast to previous Sonic games, while Yuji Uekawa redesigned the series's characters for their transition to 3D. Sonic Team attempted to demonstrate the technical prowess of the Dreamcast with realistic graphics and drew inspiration from locations in Peru and Guatemala. The soundtrack was primarily composed by Jun Senoue, who preferred rock music over the electropop of previous Sonic games.

Following its reveal at the Tokyo International Forum in August 1998, Sonic Adventure was highly anticipated and was released in Japan in December 1998, North America in September 1999, and Europe in October 1999. It received acclaim and became the Dreamcast's bestseller, with 2.5 million copies sold by August 2006. Reviewers considered Adventure a major technical advancement and praised the visuals and gameplay. Though critics noted glitches and camera problems, and reactions to the audio were mixed, they considered Sonic Adventure exceptional; some speculated that it could help re-establish Sega as the dominant console manufacturer after the unsuccessful Saturn.

Journalists have retrospectively ranked Sonic Adventure among the best Sonic games, and it is recognized as an important release in both the series and the platform genre. Many characters and concepts introduced in Adventure recur in later Sonic games. Sonic Adventure 2 was released in 2001. Adventure was ported to the GameCube and Windows in 2003 with modernized graphical choices, more challenges and additional content, while a high-definition version was released for the Xbox 360 and PlayStation 3 in 2010 and for Windows in 2011. Reviews for these releases were less positive; critics felt the game had not aged well and ran at an inconsistent frame rate.

Sonic Adventure is a 3D platform game with action and role-playing elements. Players control one of six anthropomorphic protagonists as they venture to defeat Doctor Robotnik and his robot army, who seeks the seven Chaos Emeralds and the entity Chaos. Six player characters are unlocked as the game progresses, each with their own story and attributes. Sonic the Hedgehog performs a spin dash, homing attack, and light-speed dash; Miles "Tails" Prower flies, swims, and attacks robots using his tails; Knuckles the Echidna glides, climbs walls, and punches; Amy Rose can defeat enemies using her hammer; Big the Cat is slow and carries a fishing rod he can cast; and E-102 Gamma can shoot laser beams.

At the start of the game, the player is placed in one of three Adventure Fields, open-ended hub worlds inhabited by advice-giving NPCs. The player is guided and instructed by the voice of Tikal the Echidna. Through exploration, the player discovers entrances to levels called Action Stages, some of which must be opened using keys hidden in the Adventure Field. Once the player accesses an Action Stage, they are tasked with a specific objective, which is different for each character. Sonic must reach the level's end similarly to prior Sonic the Hedgehog games; Tails must reach the end before Sonic; Knuckles must find three hidden shards of the Master Emerald; Amy must solve puzzles and avoid being caught by a robot; Big must fish for his pet frog; and Gamma must fight his way through stages using projectiles as a defense.

Some levels include minigames separate from the main story. These feature different styles of gameplay, among them rail shooting, racing, pinball, and sandboarding. Some minigames can only be accessed with particular characters. Fulfilling certain objectives allows the player to obtain bonus items. Unlocked minigames and stages the player has completed can be accessed from a Trial Mode on the title screen.

As with previous Sonic installments, players can collect golden rings, which can grant them protection from a single enemy or hazard as well as an extra life if 100 are collected. Also scattered throughout the levels are canisters containing power-ups, such as speed shoes, additional rings, temporary invincibility, and protective shields, and 1-ups. In several stages, the player engages Robotnik or Chaos in a boss fight and must deplete the boss's health meter to proceed. Point markers act as checkpoints where the character can respawn after losing a life.