The player spawns boxes from Qbby's body to traverse the 173 challenges. Each world introduces something new and the concept is rarely reused.
BoxBoy! was developed by Japanese video game company HAL Laboratory. It was a small experimental project in development while the studio was working on Kirby: Triple Deluxe and Kirby and the Rainbow Curse. To give each world a theme, they were each designed with a particular gameplay element in mind.
The development team had considered implementing a multiplayer mode for BoxBoy!. Ultimately, the feature was excluded as they believed that making a fun single-player experience should be the priority.
BoxBoy! received generally favourable reviews from critics according to review aggregator website Metacritic. Some reviewers highlighted that the early stages in BoxBoy! felt too basic, and consequently the game did not make a great first impression. GameSpot writer Justin Haywald described the simplicity of the game's initial puzzles as "almost off-putting".
However, he was satisfied with the more elaborate stages presented after progressing through the game, calling the puzzles "ingenious" and praising the intelligent use of the box-manipulation mechanic. IGN's Jose Otero complimented the game's level design and challenge posed by later stages in the game. He also liked the game's generous checkpoint system noting that it was helpful during the more difficult puzzles.
Bob Mackey of USgamer mentioned that the friendly approach, short levels, and frequent checkpoints did not detract from the game's challenge; he said that by allowing players to quickly test different solutions without having to restart a stage, HAL had managed to remove the frustration typically found in similar block-based puzzle games.
Christian Donlan of Eurogamer was surprised by the long length of the game and the amount of variety it offered given its simple premise. He enjoyed the game's pacing and felt that separating puzzles into short levels kept the game fun. Game Informer reviewer Kyle Hilliard also approved of the game's pacing. He remarked that BoxBoy!'s low price and well-designed puzzles made it entirely worth playing. VentureBeatwriter Gavin Greene agreed that the game offered a lot of content at an inexpensive price. The game's structure did draw some criticism from Jon Wahlgren, writing for Nintendo Life. He thought the game spent too much time teaching new mechanics and did not give players enough freedom to explore or experiment; as a result, he believed the game felt restricted.
According to some writers, BoxBoy!'s graphics and art resembled the style found in Game Boy titles; the reaction towards it was mixed among reviewers. While some critics described the graphics as "charming" and "stylish", others called it "sterile". The expressive animations and cute character designs were well received.