Pen Pals is a two-player mode exclusive to the Extras Mode in the Multiplayer Mode of Mario Party DS. The mode's name is a play on the phrase "pen-pals", a term for two people who often write to another usually via postal mail. In this mode, the two human players must cooperate to trap the two computer players to win. It is played on a 6x5 grid, on which the computer players move around on after rolling a Dice Block each in an attempt to escape from the human players, who roll Dice Blocks afterwards and pursue.
The human players, while holding pens, must draw their lines around a computer player to trap it and thus remove them from the game. The lines the players draw can strategically be used to corner the computer players. After the third, sixth and ninth turns, a 2-vs-2 minigame is played. Whichever team wins the minigame each obtains an extra dice block. If the players fail to catch both computer players by the end of the tenth turn, the computer team wins. If the human players catch both computer players before the turn limit has been reached, they win.