Yosumin is a tile matching puzzle game, where the player creates squares or rectangles with four identical tiles on the vertices. The main game screen shows a grid with several different shapes on each cell. The player has to click on a pair, by either selecting each tile or clicking and dragging from one to the other, to create a quadrilateral selection and make them disappear. All the tiles within the perimeter of the selected field become of the same type of the vertices tiles. Since the grid itself is a square, it is possible to remove all the tiles when every corner has an identical tile, in a move called "Yosumin!" by the game.
The main objective of most levels is to collect a number of specific tiles, shown on tabs at the left side of the screen, within a time limit. In the final levels of each stage, the goal is to collect a glass shard by creating a square/rectangle around it. Creating a set of twelve or more tiles gives a time boost, while making a sequence of squares with the same tile type removes it from the board.
Special tiles appear randomly on the board and give power-ups when collected. The Goldmin is a smiling gold square that can be used as a vertices tile for any combination, and gives extra time when collected. The Badmin is a black and spiky shape with an evil smile that shuffles all the other tiles. The Bigmin is a giant blue squiggly shape with a worried expression, taking the space of nine tiles. The Wingmin is an angry and red star-shaped tile with wings, that can be dragged and dropped anywhere on the grid. The Plusmin is a smiling and cross-shaped yellow tile, that can remove a cross of tiles. The Bossmin is similar to the Badmin, a big and black mustached tile that takes the place of nine tiles and shuffles the whole grid.
The Windows version has two modes. In the Adventure Mode, a story is told between levels by cutscenes, describing a quest to recover the broken pieces of a stained glass shrine. There's no plot in the Endless Mode, only a succession of levels of increasing difficulty.