Marasmiellus hapuuarum is a species of agaric fungus in the family Marasmiaceae. Formally described in 2011, it is known only from Hawaiian montane wet forests where it grows on aging hāpuʻu plants. The fungus produces small pinkish-buff fruit bodies up to 0.6 cm (0.24 in) in diameter, with distantly spaced narrow gills.

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