Oomycetes are water molds that are frequently isolated during a survey of waterways. Biodiversity of oomycetes in the estuary region of the Veleka River in Bulgaria was investigated in 2021. A total number of 32 isolates were derived using the baiting method. Species identification based on sequencing of the ITS region divided them into four different genera: Phytophthora (P.), Phytopythium (Pp.), Pythium (Py.) and Elongisporangium (E.). The most abundant species in the studied region was P. lacustris (sixteen isolates), followed by P. honggalleglyana (nine isolates). P. bilorbang and P. inundata were represented by only one isolate each and were recognized for the first time in Bulgaria. The genus Phytopythium was presented by two isolates that belong to different species, Pp. litorale and Pp. citrinum. In the obtained collection, the genera Pythium and Elongisporangium were represented by only one species each, Py. angustatum (one isolate) and E. anandrum (two isolates), respectively. Colony morphology of the eight collected oomycete species was characterized by cultivation of selected isolates on three different media. Potential host species of the isolated estuarine oomycetes were estimated by pathogenicity tests conducted with sixteen plants from ten diverse families. P. lacustris and P. honggalleglyana demonstrated a higher aggressiveness among Phytophthora isolates, whereas P. bilorbang and P. inundata showed less ability to infect the tested plant species. Similar pathogenicity and a potential host range for both Phytopythium species were observed. Less aggressive against analyzed plants in this study were Py. angustatum and E. anandrum.