Super Smash Bros.: Brawl is the third game in the Super Smash Brothers series of brawling games. It is a crossover fighting pitching well-known characters from Nintendo franchises, along with characters from other series, against each other. The battles take place in 41 stages with up to four players fighting at the same time. New stages can be designed using the included Stage Builder. Unlike other fighting games, players do not need to drain the opponent's health. Instead, the game is won when a player manages to knock the opponent off the stage, although some games modes only award a point to the victorious player, continuing the fight. The fighting itself does have an influence however. The character's vitality is represented by a percentage. Starting at zero, the higher it becomes, the further the character flies away when hit, improving the chances to have him disappear off the stage. Characters have certain recovery moves to get back, but each stage has a certain boundary from where no return is possible.

Each character has basic and advanced attacks (smash attacks), as well as character-specific special moves. They can also make use of items, projectiles, custom weapons and other items. This edition introduces super attacks referred to as final smashes. Activated by destroying smash balls, these feature powerful and often unstoppable attacks, regularly introducing a major transformation of the character. Other help is introduced through assist trophies (a computer-controlled character temporarily helping out) and Poké balls (a character from the Pokémon universe joins the fight).

The single-player mode is called The Subspace Emissary and features cut-scenes with all playable characters. It is team-based, and new characters will temporarily join or leave the team for which the player can determine the battle order. Other modes include Classic (a series of matches), Events with pre-determined battle conditions - also for co-op, and the Stadium mode with mini-games such as Home-Run Contest and Target Smash!

There are 35 playable characters in total (both old and new, with some removed from previous iterations), and certain characters can acquire alternate fighting styles or transformations making a total of 37 selectable. There are 23 characters available right away:

• Bowser (Mario games)
• Diddy Kong (Donkey Kong games)
• Donkey Kong (Donkey Kong games)
• Fox (Star Fox series)
• Ice Climbers (Ice Climber)
• Ike (Fire Emblem series)
• King Dedede (Kirby games)
• Kirby (Kirby games)
• Link (Legend of Zelda series)
• Lucas (Mother/EarthBound series)
• Mario (Mario games)
• Meta Knight (Kirby games)
• Olimar (Pikmin series)
• Princess Peach (Mario games)
• Pikachu (Pokémon universe)
• Pit (Kid Icarus)
• Pokémon Trainer (Pokémon universe)
• Samus (Metroid series)
• Sheik (Legend of Zelda series)
• Wario (Wario series)
• Yoshi (Yoshi series)
• Zelda (Legend of Zelda series)
• Zero Suit Samus (Metroid series)

Additionally, 14 characters can be unlocked:

• Captain Falcon (F-Zero series)
• Falco (Star Fox series)
• Ganondorf (Legend of Zelda series)
• Jigglypuff (Pokémon universe)
• Lucario (Pokémon universe)
• Luigi (Mario games)
• Marth (Fire Emblem series)
• Mr. Game & Watch (Game & Watch series)
• Ness (Mother/EarthBound series)
• R.O.B. (R.O.B. peripheral)
• Snake (Metal Gear series)
• Sonic (Sonic the Hedgehog universe)
• Toon Link (Legend of Zelda series)
• Wolf (Star Fox series)

The multiplayer mode features online play. Game types include the regular Brawl, along with a number of Group types: Special Brawl (previously known as Special Melee) with a large amount of customization options, Tourney mode (elimination-based tournaments) and Rotation (select a cast of characters for up to 16 players to switch winners and losers).