The aim of the study was to investigate diatom communities in the Kisielina River and assess water quality based on selected diatom indices: SPI, GDI, TDI, and %PT. The Kisielina River is a right-site tributary of the Vistula River, with a length of 42.34 km. During two-season studies conducted in 2023, the waters of Kisielina were characterized by neutral or slightly acidic pH and low to medium values of conductivity. A total of 600 diatom taxa were identified at 7 sites along the river's course, 44 of which were classified as dominant species. At most sites, the water was characterized by moderate ecological status (Class III) based on the SPI and GDI indices. In September, the index values slightly deteriorated (poor ecological status, Class IV), especially in the upper and near-mouth sections. The trophic index (TDI) at most sites indicated eutrophic conditions. In Kisielina, alkaliphilous and pH-neutral diatoms dominated, as well as β- and α-mesosaprobous species (Class II and III) and eutrophic or wide-spectrum trophic species (oligo- to eutraphentic).