Cortinarius lacustris is an inedible species of mushroom-forming fungus belonging to the family Cortinariaceae and found mainly in oak groves in deciduous forests.
The mushroom cap is a reddish, cinnamon brown with a diameter between . It has a cylindrical stipe that turns to a purple-black when reacting to potassium hydroxide solution exposure. The partial veil has a slight yellow colour. The ellipsoid spores measure between 8.5âÂÂ11 üm by 5âÂÂ6 üm.