Onnia tomentosa is a species of fungus in the family Hymenochaetaceae commonly known as the woolly velvet polypore.

[2] The cap is flat when young, with a blunt, rounded and yellowish-white margin, later with a slightly depressed centre and contoured in a wave pattern towards the rim, which has a rather sharp edge when old.

The stem, if present, is short and thick,[3] and dark brown to near-black; the caps may also grow directly on the base of a tree.

[4] Most commercially important conifers in Canada, including white spruce, are attacked by Inonotus tomentosus (Fr.)

[10] Seventeen white spruce plantations 43–58 years old in Ontario incurred an average of 0.7% mortality annually over a 6-year study period.