Sonic Unleashed is a platform game in which the player controls the titular Sonic the Hedgehog in two modes: fast-paced levels that take place during daytime, showcasing and using Sonic's trademark speed as seen in previous games in the series, and slower, night-time levels, during which Sonic transforms into the Werehog, and gameplay switches to an action-based, brawler style of play, in which Sonic battles Gaia enemies (those created by the main enemy in the game, Dark Gaia). Each level takes place on a particular continent, each of which is based on a real-world location. In sections of the Xbox 360 and PlayStation 3 versions of the game, the player may choose to advance the time of day in order to play as either Sonic or the Werehog; in the Wii and PlayStation 2 versions, time is advanced automatically.
Daytime levels focus on Sonic's speed, and to this extent, sees the player control Sonic through fast-moving stages containing both 2D and 3D styles of gameplay. 2D sections are reminiscent of the Mega Drive/Genesis-era Sonic games, where the player controls Sonic in a side-scrolling fashion, while 3D sections see the camera placed behind Sonic, so the player may move in all directions. In addition to moves available in past games, such as the Homing Attack, new moves are also introduced. For instance, a new sidestep feature known as the Quick Step is available, allowing Sonic to dodge left and right, and a Drift feature, which allows Sonic to make tighter turns without slowing down. The game also features a gameplay mechanic previously used in the Sonic Rush series called the Sonic Boost, which greatly increases Sonic's speed, allowing him to smash through objects, destroy enemies instantly, or even access different level paths. In the Xbox 360 and PlayStation 3 versions, an on-screen "Ring Energy" meter displays how much boost is available. The amount of boost remaining may be increased by collecting more rings, and is decreased by using the Boost. In contrast, the Wii and PlayStation 2 versions of the game represent available Boost using bars, which may be added by performing "Action Chains", destroying multiple enemies in quick succession through the use of homing attacks, or by collecting rings. Japanese game company Dimps helped design some of the stages.Nighttime levels feature slower-paced, action-oriented levels, in which Sonic transforms into the Werehog, a beast form that has great strength and stretchable arms. Gameplay here sees the player use a variety of melee and combo attacks to defeat large numbers of enemies, collect items, and move items around to advance through the level, and use his stretchable arms to cling to distant objects; simple puzzle elements are also featured. Collecting rings in these levels replenishes the Werehog's health, while a special move, Unleashed Mode, allows the Werehog to channel his energy into increasing the power of his attacks for a short time. Nighttime levels are adjusted in the Wii version to allow greater use of the Wii Remote, such that players may control the arms of the Werehog by using the Wii Remote and Nunchuk in order to grab onto objects and proceed in the levels. The Wii version of the game also expands upon the ratio of nighttime levels to daytime, with more than three times as many nighttime levels (twenty-five in all). After nighttime levels are completed, Dark Gaia Points are gained, allowing access to bonus moves.
In addition to these two gameplay types, Sonic Unleashed also features hubworlds, in which the player may reveal, as well as advance, the story of the game. Hubworlds operate differently depending on the version of the game being played; the Xbox 360 and PlayStation 3 versions feature fully interactive, explorable 3D hubworlds, similar to those in Sonic the Hedgehog and Sonic Adventure, in which townspeople may be interacted with and side quests may be undertaken, in order to gain experience or unlock items, such as artwork, videos and music tracks. In contrast, the Wii and PlayStation 2 versions feature menu-based map systems, in which players simply click on areas to talk to townspeople and find information. Within both day and nighttime levels are medals that Sonic may collect, two types of which exist: Sun and Moon. In the Xbox 360 and PlayStation 3 versions, collecting these medals allows the player to level up Sonic's Sun and Moon stats, and these must be increased to reach new stages in the game, with a certain number of Sun Medals for Hedgehog levels, and a certain number of Moon Medals for Werehog levels. Because only the Xbox 360 and PlayStation 3 versions feature playable hubworlds, these are also the only versions in which Sun and Moon Medals may be found by exploring the towns, talking to the citizens, and completing side quests. In the Wii and PlayStation 2 versions, Sun and Moon Medals are earned after completing stages and clearing their objectives. The medals are used to open up doors in Gaia gates, which can earn bonus content.