Moser Cortinarius rubellus, commonly known as the deadly webcap, is a species of fungus in the family Cortinariaceae, native to high-latitude temperate to subalpine forests of Eurasia and North America.

The mushroom is generally tan to brown all over, with a conical to convex cap 2.5 to 8 centimetres (1 to 3+1⁄4 in) in diameter, adnate gills and a 5.5 to 11 cm (2+1⁄4 to 4+1⁄4 in) tall stipe.

British naturalist Mordecai Cubitt Cooke described Cortinarius rubellus in 1887 from material collected by a Dr. Carlyle at Orton Moss near Carlisle, Cumbria.

[2] Cortinarius orellanoides was described by Henry in 1937 from mushrooms growing under bracken (Pteridium aquilinum) and beech in France, while Robert Kühner and Henri Romagnesi described C. speciosissimus (initially C. speciosus, but that name had already been given to another species of webcap) from mushrooms growing in moss among Vaccinium in pine and spruce forests of the French and Swiss Jura.

[3] Cortinarius rainierensis, described in 1950 by Alex H. Smith and Daniel Elliot Stuntz from material collected in Mount Rainier National Park in the United States,[4] is a synonym.

[2] Gasparini queried this, however, because in Cooke's original illustrations of C. rubellus, he noted that the spores were drawn as triangular or fig-shaped and were not consistent with descriptions of C. orellanoides or C.

Cortinarius rubellus has a conical to convex (partly flattening to umbonate with maturity) cap of 2.5 to 8 centimetres (1 to 3+1⁄4 in) diameter.

[16] Nicholas Evans, author of The Horse Whisperer, his wife Charlotte Gordon Cumming, and two other relatives were accidentally poisoned in September 2008 after consuming deadly webcaps that they gathered on holiday.

[17] The other three eventually received transplants after some searching for donors, despite Charlotte having only eaten three mouthfuls of mushroom; they were instrumental in setting up the charity Give a Kidney.