The Mrežnica is a river of canyons, travertine barriers and lake sections of the flow characteristic of karst rivers. In this paper, physicochemical parameters, particle size distribution in sediment and geochemical characteristics of water and sediment at a total of 4 locations in the lower course of the river Mrežnica were determined. Physicochemical parameters of water were determined in situ and in the laboratory, the particle size distribution in the sediment was determined by laser diffraction technique, and the collected water and sediment samples were subjected to multielement analysis. Using the technique of high-resolution inductively coupled plasma mass spectrometry, dissolved concentrations of elements in water samples and total concentrations of elements in sediment were determined. In all samples, the total concentrations of 11 elements were analyzed: As, Bi, Cd, Co, Cr, Cu, Ni, Pb, Se, V, Zn. According to physicochemical parameters, water samples belong to group I or II of the Croatian regulation on water classification. The obtained average element concentrations in water samples are lower or comparable to the literature values for river waters in Croatia and lower in comparison with the Croatian regulation on water classification. The results for sediment show increased concentrations for individual elements compared to the values recorded in the literature and the Croatian regulation on water classification. Possible reasons include the impact of local agriculture, industrial zones and wastewater discharge.