The game follows Mario on his quest through space to save Princess Peach from his arch-nemesis Bowser. Levels take the form of galaxies filled with a variety of minor planets and worlds while gameplay is updated with a variety of gravity effects and new power-ups.

Super Mario Galaxy was first shown at E3 2006 and enjoyed a high level of pre-release awareness. The game is one of the most critically acclaimed titles of all time, and has won numerous "Game of the Year" awards. By March 31, 2008, it had sold 6.10 million copies worldwide, making it the best-selling standalone Wii game and the third best-selling game published by Nintendo for the Wii behind Wii Sports and Wii Play, respectively.

Premise and setting

Super Mario Galaxy is set in outer space, where Mario travels from galaxy to galaxy in order to collect Power Stars with the help of the Lumas, which are earned by completing quests or defeating enemies. Each galaxy contains a number of planets and other space matter for the player to explore. The game uses a new physics system that allows for a unique feature: each celestial object has its own gravitational force, allowing the player to completely circumnavigate rounded or irregular surfaces, walking sideways or upside down, a feature seen previously in Super Mario World 2: Yoshi's Island. The player is usually able to jump from one independent object and then fall towards another one close by. Though the main gameplay and physics are in 3D, there are several points in the game in which the player's movements are restricted to a 2-dimensional axis, reminiscent of 2D Mario games.

The game's main hub is the Comet Observatory, which contains six themed domes that provide access to the forty-two galaxies available in the game. When the player first begins the game, the player only has access to a limited number of galaxies. However, as more Power Stars are collected, more galaxies become available for the player to access. Some galaxies are accessed through special means, for example, the star-shaped creatures called "Hungry Lumas" will transform into new galaxies once they are fed enough Star Bits. When 120 Power Stars are collected, the player has the ability to play through again as Mario's brother Luigi. Gameplay is slightly different while playing as Luigi, as some obstacles can be harder or easier to overcome. Once 120 Power Stars are collected with both characters, the player is rewarded two additional challenges as well as two commemorative pictures that are sent to the Wii Message Board.

There are five "Prankster Comets" that appear periodically (Speedy, Daredevil, Cosmic, Fast Foe, and Purple). When one of them comes into orbit with a galaxy, a special challenge is initiated that leads to a Power Star. The Speedy Comet challenges the player to replay an episode (usually a slightly more difficult version) within a varying time limit. The Daredevil Comet has the player replay a section of a level while shrinking Mario's health to one unit, meaning that the player must complete the objective without being damaged even once. The Cosmic Comet pits the player in a race against a doppelganger of Mario (or Luigi) to a Power Star. The Fast Foe Comet makes a galaxy's enemies more swift and thus harder to avoid. The Purple Comet, accessible only after completing the story half of the game, allots 100 purple coins (or 150, though only 100 are needed) across an area of the 15 six-star galaxies for the player to collect, sometimes within a time limit.