BOXBOY! + BOXGIRL! is a game in the BOXBOY series. Unlike the previous BOXBOY games, BOXBOY! + BOXGIRL! allows for co-op play, with one player playing as Qbby and the other and Qucy. Players who complete the game can unlock a new game mode where they play as Qudy.
The game is a puzzle platformer. Like other BOXBOY games, the player creates boxes from their body that they can use as platforms or to activate switches. The game supports cooperative play with Qucy. Each player can produce boxes separate from the other with Qucy's denoted by the small bow in the top left of each box. After completing the main game, a new mode starring Qudy will be unlocked. Qudy is taller than the other Boxpeople so, he creates tall rectangles. The game has more levels than any previous entry at 270 levels.
Completing levels and their target box usage earn the player two kinds of currency. The first is like the one the player earned in previous games and can be used to unlock music, comics, and Balloon Challenges. The selection expands as the player completes more worlds. It can also be used to get single-use powerups that help the player through levels. The second currency can only be used on the gachapon machine. Instead of unlocking accessories as the player goes through the game, the player can spend 30 of this currency to pull a random accessory.
The Balloon Challenges are time trial challenges where the player has to figure out how to pop 10 balloons in a stage as fast as possible. The levels restrict the abilities that the player gains over the course of the game. Completing these earns the gacha currency.