Co-op mode is a mode in Super Smash Bros. Brawl where the player can get another player to help him or her with 1-Player modes over Wi-Fi or directly. Locally, a second player can join in at The Subspace Emissary, All-Star Mode, Boss Battles, Target Smash, Home-Run Contest, and Multi-Man Brawl. Online, with friends, a second player can join at Home-Run Contest and Multi-Man Brawl.
 Differences from single-player versions
 Subspace Emissary
In many respects, the first player is the "lead", and the second player is a "helper", similar to the 2-player modes of Kirby Super Star. The first player chooses all the characters for the level except for one that is reserved for player 2. In stages where the camera shifts with the player, it will focus on player 1, who is also the only player who can open doors. Furthermore, if player 2 stays offscreen while the camera shifts, they are transported via a space jump to the first player. Player 2 can activate this at will by pressing the + or Start button, depending on the controller. Although player 1 chooses most of the characters, outside of each player's first pick, the remaining choices are "shared stock", in the sense that the first player to be knocked out will respawn as the next character on the list. Note that if player 2 is KO'd and no stock remains, player 1 may still be alive and can continue to play through the level. However, should player 1 be KO'd when there are no remaining stock, the game ends, regardless of player 2's status.
 All-Star Mode
If two players attempt to do All Star Mode, they will go though the normal format with a few minor changes:
- The final battle will be against two Olimars instead of just one.
- Both players must remain alive. If either is KO'd, the game ends and shows the Continue screen.
- Instead of having three Heart Containers, the players now have five, with the two extras appearing below the platforms that hold the original three.
- The game records that both characters used have completed the mode, in that both characters' Final Smash trophies will be earned and both get credit for completing the mode for challenges. However, the score earned is tracked in a unique slot reserved for co-op mode, which is not attached to any character, and so neither characters' high scores will be altered.
Note that unlike Subspace Emissary, there is no clearly designated "lead" character, in the sense that player 1's role is no different than his or her companion other than that the ending theme will match player 1's character rather than player 2. Either player can activate the portal from the rest area, and either player's demise during a fight will trigger the continue screen.
 Boss Battles
Co-op Boss Battles has a format very similar to that of 1-player Boss Battles, with a few of the same differences that All-Star Mode has:
- If one player is KO'd, the other may not carry on alone.
- There are five Heart Containers.
- There is only one final score rather than two to be recorded onto the characters.
Like All-Star mode, there is no distinction between the roles of each player. Either player's death will trigger the mode's end, and either player may activate the teleporter in the Rest Area.
 Target Smash, Home-Run Contest, and Multi-Man Brawl
Target Smash, Multi-Man Brawl and Home-Run Contest follow the same format as in one-player mode, except for the fact that there are two people playing rather than one. Challenges for Target Smash and Home-Run Contest modes can be earned in co-op, although records for all three are saved separately. Like All-Star Mode and Boss Battles mode, there is no distinction between either players' roles, and either player being KO'd will trigger a loss.