Cosmoem is a Pokémon in the shape of a translucent, dark blue orb with a starscape pattern set in a golden shell-like structure. The main portion of the shell is a flat oval around the center of the orb. Eight more flat, half-oval projections surround the orb in an asterisk-shape. At the center of the main body is a much smaller black core. The upper half of the core has a thin blue line on the front, back, and each side, while the lower has pink lines. On the front are two orange semicircles, which appear to be closed eyes. The overall shape of this Pokémon also resembles an eye.
Encased in its sturdy, cocoon-like shell, Cosmoem is unable to move. In Pokémon Refresh, it cannot even eat. It grows as it absorbs light, and its body is warm to the touch. In ancient times, it was worshipped as the "cocoon of the stars" by the people of Alola.
Cosmoem has characteristics of both a stylized sun and a stylized moon, having rays and a pair of crescents, respectively. Its enormous weight despite its extremely small size shares similarities with super-compressed astronomical bodies such as black holes and neutron stars. Given its description, it seems to be an embodiment of star formation in general. It may also be based on the idea of the world egg. When viewed from the front, its shape resembles an eye.