Abstract: To investigate interzonal runoff exchange and its controlling factors during urban pluvial flooding, a typical urban area in Weinan City, Shaanxi Province, China, was selected as the study area. A coupled one-dimensional–two-dimensional hydrological–hydrodynamic model was developed, and a tracer-based source attribution method was used to label the effective rainfall generated within each drainage zone. Inundation and drainage processes under a range of rainfall magnitudes were simulated to quantify the contributions of cross-zone runoff and to identify the governing factors and spatiotemporal characteristics of surface runoff exchange. The results showed that: (1) pronounced runoff exchange occurred when the rainfall depth reached 52.08 mm. The exchange process was primarily governed by topography and drainage-network capacity, while the onset thresholds varied considerably among the drainage zones. (2) Runoff exchange exhibited pronounced spatial propagation and temporal lag. Topography controlled the direction of runoff transfer, whereas the drainage network regulated the exchange volume. External runoff originating from Zone Z2 contributed up to 57.7% of the ponding in adjacent zones, and its peak occurred after the rainfall peak. (3) Runoff exchange showed substantial spatial heterogeneity. Zone Z2 was the primary source of external runoff and exhibited the fastest response, whereas Zone Z3 was the principal receiving zone. At a representative ponding location, the peak contribution of external runoff reached 66.64%. These findings clarify the mechanisms and spatiotemporal patterns of runoff exchange across urban drainage zones and provide a scientific basis for identifying runoff sources, optimizing drainage-zone delineation, and improving boundary management for urban flood mitigation.