During a survey of oomycetes in ornamental plants carried out at the University of Aberdeen in 2014–2015, Pythium kashmirense was isolated from a specimen of Viburnum plicatum ‘Lanarth’, the first report of this oomycete in the UK (and in Europe). Pathogenicity of a Py. kashmirense isolate was examined using a range of plant species. Inoculations were carried out under controlled conditions in the absence of other Pythium and Phytophthora species, on Glycine max (soya bean), Phaseolus vulgaris (common bean), Lupinus angustifolius (blue lupin), Cucumis sativa (cucumber) and Viburnum opulus. The majority of inoculations caused pre-emergence damping-off, as well as seed rot and root rot. In the in vitro assays, germination rates (%) of soya bean and blue lupin seeds were less than 50%; in the in vivo inoculations on plants, over 50% of soya bean, blue lupin and common bean plants died; in contrast, cucumber plants showed lower susceptibility in pathogenicity tests, with an approximately 80% germination rate in in vitro tests, and 25% dead plants in the in planta inoculations. Inoculations carried out on root systems of Viburnum opulus caused severe necrosis and root rot. Little research was previously conducted on pathogenicity of Py. kashmirense and its relationship with losses in crop yield and quality. The present study showed varying virulence on the different plant species tested after inoculation with Py. kashmirense. Despite the lack of clear host specialization, infection by Py. kashmirense decreased seedling survival and health of plants in a range of important agricultural and ornamental plant species.