Identifying nonpoint phosphorus (P) sources in a watershed is essential for addressing cultural eutrophication and for proposing best-management solutions. The oxygen isotope ratio of phosphate (δ18OPO4) can shed light on P sources and P cycling in ecosystems. This is the first assessment of the δ18OPO4 distribution in a whole catchment, namely, the Yasu River Watershed in Japan. The observed δ18OPO4 values in the river water varied spatially from 10.3‰ to 17.6‰. To identify P sources in the watershed, we used an isoscape approach involving a multiple-linear-regression model based on land use and lithological types. We constructed two isoscape models, one using data only from the whole watershed and the other using data from the small tributaries. The model results explain 69% and 96% of the spatial variation in the river water δ18OPO4. The lower R2 value for the whole watershed model is attributed to the relatively large travel time for P in the main stream of the lower catchment that can result in cumulative biological P recycling. Isoscape maps and a correlation analysis reveal the relative importance of P loading from paddy fields and bedrock. This work demonstrates the utility of δ18OPO4 isoscape models for assessing nonpoint P sources in watershed ecosystems.