Collybia cookei is a species of fungus in the family Tricholomataceae, and one of three species in the genus Collybia. It is known from Europe, Asia, and North America. The fungus produces fruit bodies that usually grow on the decomposing remains of other mushrooms, like Meripilus giganteus, Inonotus hispidus, or species of Russula; occasionally fruit bodies are found on rich humus or well-decayed wood. The fungus produces small white mushrooms with caps up to 9 mm (0.35 in) in diameter, supported by thin stems that originate from a yellowish-brown sclerotium. The mushroom is difficult to distinguish from the other two species of Collybia unless an effort is made to examine the sclerotia, which is usually buried in the substrate. The edibility of the mushroom has not been determined.