Dissolved inorganic phosphorus (DIP) is a key limiting nutrient in freshwater ecosystems, and its excessive input poses a serious threat to lake water quality and ecological stability. Previous studies have primarily focused on identifying DIP sources in surface waters such as rivers and lakes, while systematic understanding of the spatiotemporal distribution and source contributions of groundwater DIP remains limited. In this study, the Taihu Basin was selected as a representative case. Groundwater samples were systematically collected during the normal, wet, and dry water period in 2024 to analyze DIP concentrations, hydrochemical characteristics, and phosphate oxygen isotope compositions for source apportionment. The results showed that groundwater levels across the basin generally exhibited a spatial pattern of higher values in the west and lower values in the east, with a central depression zone. During the wet water period, groundwater levels were higher and primarily discharged into Lake Taihu, whereas during the dry water period, groundwater levels declined and the flow direction reversed toward discharge-dominated conditions. Spatially, groundwater DIP concentrations were higher in the central and southern regions of the basin and lower in the western and eastern regions. Temporally, DIP concentrations followed the order wet > normal > dry water period. Hydrochemical analyses indicated that groundwater was mainly affected by agricultural activities and domestic sewage during the wet and normal water period, while industrial activities exerted stronger influence during the dry water period. Source apportionment based on phosphate oxygen isotopes revealed that, during the normal season, the contributions of industrial effluents, domestic sewage, and agricultural wastewater were comparable, accounting for 34%, 31%, and 30%, respectively, while forest runoff contributed the least (5%). During the wet water period, agricultural wastewater became the dominant source, accounting for 37%, whereas domestic sewage and industrial effluents decreased to 28% and 25%, respectively, and forest runoff slightly increased to 10%. In contrast, during the dry water period, the agricultural contribution further increased to 42%, followed by domestic sewage (31%), while forest runoff rose significantly to 18% and industrial effluents declined to 9%. Overall, this study provides the comprehensive characterization of the spatiotemporal patterns and seasonal variations in groundwater DIP sources across the Taihu Basin. The findings offer new insights into identifying dominant groundwater phosphorus sources and provide a scientific basis for developing differentiated strategies for water quality management and eutrophication control in large lake basins.