Cladonia gracilis or the smooth cup lichen[2] is a species of fruticose, cup lichen in the family Cladoniaceae.

[3] German botanist Carl Ludwig Willdenow transferred it to the genus Cladonia in 1787.

[4] In North America, it is known colloquially as the "smooth Cladonia".

[5] The Cladonia gracilis group is a monophyletic group of species that all are morphologically similar to C. gracilis.

In this group, the delimitations of species is difficult due to the morphological similarity between taxa, and the fact that many of the characters used to classify species are influenced by environmental factors such as light exposure, temperature or humidity.