Rivers, streams and small water bodies constitute important refugia for rare and poorly known diatom taxa, their diatom diversity remains insufficiently documented yet. The aim of this study was to document the occurrence, spatial distribution and selected ecological characteristics of diatom taxa newly recorded in Poland. The study was conducted between 2017 and 2025 at selected sites in rivers and streams of southern and south-eastern Poland, as well as in small water bodies located in the UMCS Botanic Garden in Lublin and in the village of Ruda. Water temperature, pH and electrolytic conductivity were measured at the investigated sites, and available physicochemical data were used to characterise the habitats of the recorded taxa. Nineteen diatom taxa previously unrecorded in Poland were identified. Most taxa were recorded at low abundance, with several represented by single valves, which limited the possibility of defining their ecological preferences. The highest number of new records was associated with the Kisielina River, where several taxa occurred under eutrophic, alkaline or potentially wastewater-impacted conditions, including Sellaphora saprotolerans, Neidium quadrinflatum, Gomphonella linearoides and Tryblionella aerophila. Particularly noteworthy was the record of Frustulia pumilo, previously known mainly from South America, which considerably extends its documented geographical range. The results expand the known diatom flora of Poland by 19 taxa and demonstrate that rare and previously overlooked species occur in a broad spectrum of aquatic habitats. The occurrence of several new records in the Kisielina River indicates that eutrophic and anthropogenically influenced waters may harbour a substantial, but still underestimated, diversity of rare diatoms. At the same time, the low abundance, microhabitat specificity and taxonomic difficulties associated with several taxa, often requiring SEM examination, highlight the importance of detailed taxonomic surveys for improving knowledge of diatom diversity and distribution.