Smash Hit involves a steadily-moving view through a decorative, colorful passageway. As is typical with these types of often-called "mindless" games, the music changes along with the level, as the pace is fixed and the player doesn't slow down due to anything except "losing" the game. The player has an inventory of metal balls (of which the player begins with 25) that they can tap the screen to aim and shoot. The player must be careful not to run out of these balls, for if they do, the game is over and they must either 1) start again from the beginning (free version) or 2) start at the beginning of the checkpoint (premium version). Obstacles, many of which may be in motion, may get in the way, and if an obstacle is hit, the player loses 10 balls. Many of these obstacles are made of glass and can be smashed (hence the name Smash Hit), but some are not and require the hitting of buttons to be moved out of the way. There are power-ups, such as not depleting the player's ball supply for a short time, turning all their balls explosive for a short time, or slowing time down for a short time.
There are also small, blue polyhedra scattered throughout the map. By hitting a square pyramid with a ball, the player receives 3 balls (net gain +2); by hitting an octahedron, 5 balls (net gain +4); and by hitting a 3D star, 10 balls (net gain +9). If the player hits 10 of these polyhedra in a row without missing any, they upgrade their multi-ball status. The player must hit 40 in a row to get to the maximum multi-ball from the single-ball default. The maximum amount of balls the player can shoot with a single tap is 5. If the player hits an obstacle or miss any of the ball-gaining targets, they lose all their multi-ball status and must regain it.
Over time, the levels get more and more challenging. If the player completes all the checkpoints, there is an "endless mode" at the end, where the goal is to survive as long as possible repeating the same course over and over. For every 1% through a checkpoint, 10 is added to the score. At the end of one checkpoint, the player will have gained 1,000 points. In endless mode, the game still counts the points, except it is the same course over and over rather than different courses. Some of the highest scores in the world are in the ballpark of 20,000; a dozen of players have scored more than 30,000; the average experienced player will average 14,000-16,000; and the occasional player will average 4,000-8,000. A first-time player can sometimes break 1,000.
There is no "winning" the game. The player is guaranteed to "lose" (run out of balls) eventually, the object of the game being how long it took for the player to lose, rather than if they won.