The player controls the Prince as he rolls the katamari around houses, gardens, and towns in order to meet certain parameters set by the King of All Cosmos. The player uses the two analog sticks on the DualShock controller in a manner similar to the classic arcade game Battlezone to control the direction the katamari rolls. Other controls can be triggered by the player to gain a quick burst of speed, flip the Prince to the other side of the katamari, and more.
Objects that are smaller than the katamari will stick to it when the player comes into contact with them, while greater objects can be hurdles; colliding at high speed with any may cause objects to fall off the katamari, slowing the player's progress. The game uses size, weight, and surface area to determine if an object will stick to the katamari. This allows slender objects, such as pencils, that are wider than the katamari, to be picked up, and these will alter how the katamari rolls until more objects are picked up. Animals such as cats will chase the katamari, knocking things from it, but once the katamari is great enough, it will scare the animals away, and they can be rolled up once they are chased down. As objects stick to the katamari, the katamari will grow, eventually allowing objects that were once hurdles to be picked up, and creating access to areas that were formerly blocked. In this manner, the player might start the game by picking up thumbtacks and ants, and slowly work up to the point where the katamari is picking up buildings, mountains, and clouds.
The typical mission given by the King of All Cosmos is the "Make a Star" mode, where the player needs to grow the katamari to a specific size within a given timeframe. Other missions have more specific collecting rules, such as collecting as many items (swans, crabs, pairs) within a given time, or collecting the greatest item possible (such as a cow or bear). The player can attempt a score attack mode for any level, in which they would try to make the greatest katamari possible in the time allotted. Certain levels can unlock an "eternal mode" by creating a katamari twice the goal size. In eternal modes, the player can explore the level with no time limit.
Levels feature two secret items that can be found. The first item is a royal present that contains an object that the Prince can wear. Most gifts are non-functional, but one includes a camera that can be used to take in-game screenshots. The other secret item is a cousin of the Prince, which, once rolled up in main gameplay, can be used as a character in the various multiplayer modes. The game also tracks which objects the player has collected at any time, allowing them to review all the various objects within the game.
In the two-player mode, a player can choose to play as either the Prince or one of his numerous Cousins. The screen is split vertically; player one is on the left, and player two is on the right. Players compete simultaneously in a small arena to collect the most objects within three minutes. The playfield is replenished with new objects periodically. Players can ram into each other, knocking items from their opponents' katamaris, and if one player leads by a fair amount, then it is possible to roll up the opponent's katamari.