Lactarius argillaceifolius is a species of fungus in the family Russulaceae. The mushrooms have convex to flattened drab lilac-colored caps that are up to 18 cm (7.1 in) wide. The cream-colored gills are closely spaced together and extend slightly down the length of the stem, which is up to 9 cm (3.5 in) long by 3.5 cm (1.4 in) thick. The mushroom produces an off-white latex when injured that stains the tissue brownish.

The variety megacarpus, with caps up to 27 cm (11 in) wide, occurs under coast live oak and tanoak in the Pacific Coast states and Baja California. Variety dissimilis, described on the basis of a single specimen from South Carolina, differs from the main form in the microscopic structure of the cap cuticle. The European species L. trivialis is similar, but can be distinguished by differences in habitat and color differences in the gills, cap, and latex.

L. argillaceifolius is found in eastern North America and has also been found in pine plantations in Brazil, where it is probably an introduced species. Its edibility is unknown but is possibly poisonous.

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