Abstract: Over half of China’s steep-slope croplands, strictly defined as agricultural lands situated on topographic gradients steeper than 25 degrees, are densely concentrated within the complex mountainous and hilly terrains of Southwest China. The spatiotemporal dynamics and developmental trajectories of these highly vulnerable marginal lands exert a profound, unignorable influence on the dual national strategic imperatives of safeguarding food security and advancing ecological environmental construction. Clarifying the precise spatiotemporal evolution characteristics of these steep-slope croplands, alongside a rigorous identification of their underlying influencing factors, is critically essential for establishing a robust scientific basis to refine regional land management policies. To address this necessity, this study constructed a specialized steep-slope cropland density indicator by integrating high-resolution 30-meter digital elevation models with comprehensive annual land cover datasets spanning a twenty-five-year period from 2000 to 2025. Utilizing a meticulously selected standard grid unit to balance macroscopic regional trends with micro-level spatial heterogeneity, the research employed an array of advanced spatial analysis techniques—including global spatial autocorrelation, cold and hot spot analysis, and detailed land-use transition matrices—to systematically depict the overarching spatiotemporal patterns and the complex, bidirectional inward and outward conversion characteristics of steep-slope croplands across the southwestern region. Furthermore, a Random Forest machine learning algorithm was innovatively applied at the county administrative scale to quantitatively parse out the primary driving factors controlling the static spatial distribution and the dynamic temporal shifts of these croplands among a multitude of natural and anthropogenic variables. The empirical findings reveal several critical insights. First, steep-slope croplands in Southwest China are widely and prominently distributed across diverse geomorphological landscapes at elevations predominantly below 3000 meters. The region maintains an average steep-slope cropland density of 2.71%, which exhibits a highly significant degree of spatial agglomeration, with distinct hotspots concentrated along major mountain ranges. Throughout the 2000–2025 observational period, the overarching density of steep-slope cropland across the entire study area experienced a gradual but definitive net decline from 2.84% down to 2.52%. However, this macroscopic reduction was characterized by intense spatial heterogeneity, presenting a distinct geographical pattern defined by a pronounced, continuous decline in the northern territories, starkly contrasted by a highly fragmented, staggered, and interwoven pattern of simultaneous localized expansion and contraction throughout the central and southern zones. Second, an in-depth analysis of the land-use transition matrices from 2000 to 2025 indicates that the outward conversion of steep-slope croplands was directed primarily toward ecological restoration, with the proportions transferred into forests, shrublands, and grasslands accounting for 53.84%, 42.42%, and 3.56%, respectively. In contrast, the reverse inward conversions from these ecological lands into steep-slope agricultural use were predominantly sourced from forests (69.94%), followed by shrublands (15.46%) and grasslands (14.40%). This net directional shift conclusively demonstrates that the overarching land-use conversions involving steep-slope croplands during this quarter-century period have been highly beneficial for mitigating ecological degradation and enhancing the overall habitat quality of the region. Third, the Random Forest predictive modeling demonstrated that topographic constraints and foundational land-use factors consistently ranked at the forefront of variable importance, maintaining a stable and dominant influence on steep-slope cropland density over time. Conversely, the explanatory power exerted by standard socio-economic variables and ecological quality indicators was relatively weak and exhibited frequent temporal volatility. Ultimately, qualitative assessments confirm that overarching macro-policies function as the most crucial underlying driving forces dictating these spatiotemporal transformations. These comprehensive research results significantly enrich the theoretical and empirical understanding of the spatiotemporal configuration of steep-slope croplands in Southwest China, thereby providing a fundamental, data-driven reference for optimizing territorial spatial development and protection patterns, and for tailoring more precise, differentiated steep-slope cropland governance strategies in the future.