Abstract: The Loess Plateau is one of the most critically important dryland regions in northern China. There is a pronounced mismatch between the seasonal distribution of rainfall and the water requirements of its crops in agricultural production. This study aims to assess the potential implementation zones, suitability levels, and yield improvement of the rainwater harvesting and supplemental irrigation (RHSI). Remote sensing and ground observation data were integrated from 2000 to 2018. Spatiotemporal patterns of precipitation and evapotranspiration were analyzed to evaluate the fundamental feasibility of the RHSI deployment across the plateau. The Soil and Water Assessment Tool (SWAT) was specifically employed to simulate the volume of harvestable rainfall runoff available for capture. While the water requirements were calculated using the standardized Penman–Monteith equation. Subsequently, a reinforcement learning model was applied to optimize the essential spatiotemporal synchronization between the available water supply (represented by simulated runoff) and the identified crop water demand (quantified as water deficits). This fundamental synchronization was also obtained to accurately delineate the potential RHSI implementation zones. The optimal application timing was determined to estimate the supplemental irrigation volumes. The results revealed that a pronounced imbalance of the seasonal water was characterized by an average regional water deficit of 40.32 mm from December to June, whereas a significant surplus of 45.15 mm was accumulated from July to November annually. High potential utilization of the practical rainwater was predominantly concentrated in the distinct hilly–gully region (D) and the eastern high-plateau areas, specifically east of the Liupan Mountains. The annual crop water demand averaged 518 mm. There was a spatial gradient decreasing progressively from approximately 600 mm in the northwestern areas down to about 400 mm in the southeastern areas. Crucially, 78.7% of the total annual crop water demand occurred during the vital spring and autumn growing seasons. The average annual runoff depth reached 45 mm. The Zones D and F (the southern high-plateau subregion) contributed to a substantial 39% of the total runoff volume. Approximately 3.76 million hectares of the existing rainfed farmland were identified as suitable for the practical RHSI implementation, which was distributed in the key zones of D (hilly–gully), F (southern high-plateau), and E (rocky mountainous zones). The average annual supplemental irrigation volume necessary per unit area reached 719 m³/hm². This irrigation was typically applied an average of 3.45 times annually, with a single application event averaging 204.30 m³/hm² in volume. Irrigation scheduling was highly concentrated during the critical April–June period. Specifically, the peak crop water deficits occurred after irrigation. There were regional differences in the scheduling patterns. For instance, 85% of irrigation events within the Ningxia irrigation zone (C) occurred during April–May, whereas more than 85% of irrigation events were concentrated in the southern hilly–gully subregion (D2) in June. Interannual variability also demonstrated that the RHSI implementation was potentially expanded to cover 4.04 million hectares during wetter years. There was a decrease in the average application volume of 142.50 m³/hm² per event. Conversely, the suitable area increased slightly to 4.41 million hectares during drier years. There was a further reduced average application volume of 119.10 m³/hm² per single irrigation. The crop yield responses confirmed that the RHSI significantly enhanced the agricultural productivity. The regional average yield increased, ranging from 30% to 80%. Zones D and F demonstrated strong performances in the implementation areas. The average yield was improved by 49.16% and 41.83%, respectively. The maximum potential yield increased at the high levels. Specifically, there was the 97% for rapeseed and 95% for spring wheat, particularly in the hilly–gully zones. Overall, the RHSI can be expected to substantially mitigate the seasonal water shortages in the Loess Plateau.