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To clarify the pathways and origins of dissolved nitrate (NO₃⁻) in rivers flowing into Lake Biwa, Japan, three types of coordinated surveys of concentrations and isotope compositions of NO₃⁻ were conducted: (1) synoptic river sampling of 32 representative inflow rivers, (2) two rivers in catchments with different land uses, and (3) headwater catchment samplings. The data set used in this paper is the first comprehensive collection of isotopic composition of NO₃⁻ in rivers of a large‐scale basin in Asia. While the NO₃⁻ spatial concentration pattern showed significant seasonal variation, the spatial pattern of the δ¹⁵N of dissolved NO₃⁻ was more consistent throughout the year. The was significantly positively correlated with the population density of each catchment. A mass balance model assuming the and the flow rate of sewage effluent was developed. The model simulated the relationship between the population …