Abstract: Species in the genus Phytophthora have been often associated with dieback and declines of oaks worldwide. Based on its phylogeny, the genus has evolved in 12 clades, some of which comprise species strictly related to water environment. Most of the species associated to oak decline are soilborne and their detection have been undertaken mainly through the isolation from soil samples and symptomatic tissues of infected trees, and only in some cases analysing water samples. Therefore, this study was carried out for monitoring the diversity and distribution of Phytophthora spp. in different Sardinian waterways located in oak forest ecosystems showing declining symptoms associated with Phytophthora spp. Ten different sites were selected, within which three traps containing fresh leaves of different plant species were used as baits by floating them over the river water. After 5-7 days, the traps were collected and transported to the laboratory for Phytophthora isolation using a selective medium. The identification was based on morphological and molecular analyses. In total 11 different Phytophthora spp. were isolated. Phytophthora gonapodyides turned out to be the most frequently isolated species, followed by P. bilorbang, P. lacustris, P. plurivora, P. pseudocryptogea, P. hydropathica, P. chlamidospora, P. × cambivora, P. parsiana, P. × alni and P. sp1. Surprisingly, P. cinnamomi was never recorded from water, although it was present in the soil at some sites. This study showed the high diversity of Phytophthora spp. in aquatic environment, including the detection of previously unrecorded species in oak forests.