Niesslia is a genus of fungi in the family Niessliaceae. It was circumscribed by German mycologist Bernhard Auerswald in 1869, with Niesslia chaetomium assigned as the type species.

These organisms, which are barely visible to the naked eye, are found in decaying plant matter and are parasites of lichens, other fungi, or nematode eggs. They belong to the ascomycetes and in their teleomorphic (sexual) stage they form distinctive dark brown shiny fruiting bodies with spines. The fruiting body structures are a flask-shaped type called perithecia where the spores escape through ostioles. The asexual anamorphic stage was given the genus name Monocillium, now considered a synonym.

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