Abstract: Understanding the spatiotemporal heterogeneity in the effects of climate change and dryland to paddy field conversion on cropland soil organic carbon (SOC) is essential for improving cropland carbon sequestration capacity and soil quality. Taking Harbin City, a typical black soil region, as the study area, this study used a geographically weighted regression (GWR) model to characterize the spatial heterogeneity of the effects of climate change, dryland to paddy field conversion, and their interactions on SOC, and applied variance partitioning analysis (VPA) to quantify the relative contributions of different factors. The results showed that: 1) cropland SOC generally declined from 1999 to 2019, with an average change of −0.73 kg/m²; 2) dryland to paddy field conversion generally promoted SOC accumulation, with higher carbon sequestration potential in the northern and southern regions, whereas paddy field to dryland conversion significantly inhibited SOC growth, with a higher risk of SOC loss in the southwestern region and parts of the central region. The effect of temperature change on SOC showed a spatial pattern characterized by inhibition in the northern and southern regions and promotion in the central and western regions; 3) the interaction between temperature change and paddy field to dryland conversion generally inhibited SOC accumulation, with the strongest negative effect in the northern region. In contrast, the interaction between temperature change and dryland to paddy field conversion showed a spatial differentiation pattern, with promotion in the northern region and inhibition in the southern region; 4) compared with single climatic factors, the interaction between climate change and dryland to paddy field conversion had the strongest explanatory power for the spatial heterogeneity of SOC and represented the dominant influencing mechanism. These findings provide a scientific basis for the rational utilization of cropland and the development of carbon sequestration enhancement measures in black soil regions.