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Paxillus involutus, commonly known as the brown roll-rim or common roll-rim, or poison pax, is a poisonous basidiomycete fungus, previously considered edible and eaten widely in Eastern and Central Europe. It had been recognized as causing gastric upsets when eaten raw, but was more recently implicated in a potentially fatal immune hemolysis in those who had consumed the mushroom without ill-effects for years. It often grows near edible mushrooms as well which makes it harder to identify by amateur mushroomers.

It is widely distributed across the Northern Hemisphere and has been accidentally introduced to Australia, New Zealand, and South America; it is likely to have been transported in soil of European trees to those countries. Various shades of brown in color, the fruiting body resembles a brown wooden top and may be found in deciduous and coniferous woods, and grassy areas in later summer and autumn. The cap bears a distinctive inrolled rim and decurrent gills which may be pore-like close to the stipe. Although it has gills, it is more closely related to the pored boletes than to typical gilled mushrooms.