A Katamari is a magical device created by The King of All Cosmos. It is a multicolored, bumpy ball that has the ability to collect objects on its surface. Any objects that come in contact with the Katamari gain this adhesive ability until they are knocked loose. A sharp knock by a moving object or trying to roll up an object too large for the Katamari will cause objects to fall off. At the start of a level they can vary in size and shape, from a few centimeters to a few meters wide.
Apparently, any object can be turned into a Katamari, as on more than one occasion, the King has used other, more unusual Katamaris to collect objects with, including an origami ball and a sumo wrestler. The sumo wrestler is used in Katamari Forever and anything food related that is rolled up by the "Katamari" is eaten by the wrestler, causing him to grow. Another roll in Katamari Forever allows the Katamari to hold water. The sprinkler Katamari is then used to clear sand from a very dry planet. Although the Katamari has the new skill of absorbing water, it can still roll up the Present and Pu who are found within the level. The process of creating a Katamari is unclear, although it seems to be an ability shared by the Royal Family. The Moon, the Earth, and even the Sun have been used as Katamaris by the Prince and King.
Katamaris are used to replace the stars in the Cosmos. The number of objects collected is not as important as the size of the Katamari. A Katamari with a large number of small objects may not pass the size requirement, while one with fewer, larger objects will. Once a Katamari has reached a certain size, the King can then turn it into a star or planet. The required size seems to be arbitrary, as a larger Katamari may be used to make a smaller star.
In Touch My Katamari however, The king says "The Katamari doesn't have to be round. We thought you would have known that."
- Although most believe that The King created the Katamari, it is said in We Love Katamari that an orange-colored animal invented the Katamari. His name is Dr. Katamari, and where it was said was in this fans' collection description.
- Note that the color of the katamari differs with the current level. (Ex: Make Taurus's Katamari is colored with cow spots.)
- All Cousins roll a certain Katamari in multiplayer depending on their color and style.
- It's currently unknown if Katamaris are weightless or if The Prince is just so strong that he can push planet sized objects.