Background: Japan is one of the most rapidly ageing societies in the world and has experienced sustained population decline since 2008. Anticipating how these demographic changes will influence future inpatient demand is essential for planning hospital capacity and health care delivery. Methods: This repeated cross-sectional study used nationwide aggregated data from Japan’s Patient Survey (1999–2023) and population data from the national census and official projections. We modelled age-, sex-, and prefecture-specific inpatient utilization using three approaches: age–period–cohort models, statistical trend models (including linear, log-linear, and spline specifications), and conservative scenario models. Projected utilizations were applied to municipal population projections using indirect standardization, and results were aggregated to prefectural and national levels for 2025–2050. Findings: Between 1999 and 2023, inpatient utilization and the number of inpatients declined despite population ageing. Most models projected continued declines in utilization across age groups. When combined with population projections, total inpatient was projected to decrease toward 2050. Conservative scenarios suggested temporary stabilization or slight increases around 2030, but long-term trends remained downward. Spatial analyses showed increasing concentration of demand in metropolitan and regional centres, while many peripheral municipalities were projected to experience very low inpatient volumes. Interpretation: In Japan, inpatient demand is likely to decline and become increasingly concentrated in urban areas despite continued ageing. These findings highlight the need for region-specific strategies to optimize hospital capacity and adapt health systems to population decline.