Cortinarius sublargus is a species of fungus in the family Cortinariaceae native to Australia. It was described in 1928 by John Burton Cleland from the Mount Lofty Ranges. Cleland also described Cortinarius radicatus in 1933 from material collected in Willunga Hill, Waitpinga, Mount Lofty, Mount Compass, and Kinchina, Though Cleland regarded them as distinct, later authorities determined them to be the same species. The latter name turned out to be a homonym, having already been given to a different species.

William Murrill named an American species Cortinarius sublargus in 1939, though renamed it C. largiformis in 1944 as it too was a homonym.

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