Mycetinis opacus is a species of agaric fungus first described in 1849 by Miles Joseph Berkeley and Moses Ashley Curtis as Marasmius opacus.

[2] Andrew Wilson and Dennis Desjardin transferred it to Mycetinis in 2005.

[3] It is found in North America (and rarely in Japan) growing especially on dead Rhododendron material, but also on debris of oak, pine, and eastern hemlock.

The cap reaches only to about 2 cm diameter and it has conspicuous pale mycelial cords.

Unlike some other Mycetinis species, it does not smell of garlic.