Phlegmacium ponderosum, also known as the Ponderous Cortinarius, is a species of mushroom producing fungus in the family Cortinariaceae.

[2] In 2022 the species was transferred from Cortinarius and reclassified as Phlegmacium ponderosum based on genomic data.

It often has an olive metallic tinge, and the surface is viscid, often with small rusty brown scales.

[4] It has a slimy yellow universal veil, and the cortina leaves a rusty brown hairy area on the upper stalk.

[4] The specific epithet ponderosum (originally ponderosus) is named for the Pinus ponderosa trees which Smith observed the mushrooms growing under.