Revealing the associated effect and spatial interaction mechanism between urban expansion and water pollutant emissions is an important issue for studying the water environmental effects of urbanization. The Yangtze River Delta (YRD) has been experiencing rapid urbanization at an annual rate of nearly 1 percent from 2011 to 2015. Rapid urbanization not only increased the impervious surface of a city and changes the structure of the regional ecological environment but also introduced high-density industrial production and urban living activities that accelerated the emission of pollutants. Taking the YRD as an example and selecting COD and NH₃-N as two characteristic pollutants, this study described the temporal and spatial characteristics of water pollutant emissions at the county level from 2011 to 2015, optimized the spatial Durbin model (SDM) to estimate the associated effect of urban expansion and water pollutant emissions, and quantitatively measured the spatial interaction between them through direct and indirect effects. The results show that (1) the water pollutant emissions in the YRD have decreased significantly from 2011 to 2015, and the high-intensity emission pattern changed from a contiguous to a scattered distribution, which were concentrated in urban periphery counties and coastal and riverside counties. (2) The associated effect of urban expansion and water pollutant emissions was significant and stable. In 2011, every 1% increase of urban expansion led to 0.293% and 0.319% increase in COD and NH3-N emissions in the YRD, respectively. By 2015, the increase rates of COD and NH3-N emissions reached 0.300% and 0.380%, respectively. The counties with high expansion and high emissions were concentrated in Shanghai and its surrounding areas and northern Jiangsu. (3) The coastal and riverside areas were the layout directions that urban expansion usually chose. Under the control of pollution distance cost and environmental regulation intensity, the farther the distance from the coastline and the Yangtze River, the lower the water pollutant emission intensity. (4) The spatial interaction between urban expansion and water pollutant emissions had two sides. Due to the siphon effect, every 1% increase in the scale of urban expansion resulted in 0.274% and 0.368% increases in local COD and NH₃-N emissions, respectively, while emissions in the adjacent counties synchronously decreased by 1.017% and 0.650%, respectively. (5) To break the association and spatial interaction between urban expansion and water pollutant emission and alleviate the environmental stress on the YRD, in addition to delimiting an urban expansion boundary and strictly restraining the scale of expansion, improvement in the regional environmental carrying capacity through urban water pollutant treatment facilities and pipe network construction is urgently needed.