Competing fungi in white button mushroom (Agaricus bisporus, champignon) cultivation causes significant episodic losses. One of these competing fungi is known as “smoky mould”. It owes its name to the production of high numbers of spores after the disturbance of compost, which resembles smoke. We investigated strains isolated from smoky mould cases in the Netherlands, UK and Canada and show that these outbreaks were caused by a new Penicillium species, named P. hermansii sp. nov. (type strain CBS 124296T). Several Penicillium species are reported to cause smoky mould. However, we so far have no indications that smoky mould is caused by other Penicillium species than P. hermansii. This species belongs to section Exilicaulis and differs from other Penicillia by its slow growth rate on Czapek yeast agar (CYA) and malt extract agar (MEA) and its inability to grow on CYA supplemented with 5% salt and CYA and MEA incubated at 37 °C.