This study analyzes the impact of demand forecasting on inventory management efficiency, focusing on the implementation of the ARIMA method in the context of a manufacturing company in Indonesia. Using a quantitative case study approach, the research explores the relationship between forecasting accuracy and key inventory performance metrics. Results indicate that improved forecasting accuracy significantly enhances inventory efficiency, with the ARIMA method outperforming traditional methods. The implementation of ARIMA resulted in a 36.8% increase in Inventory Turnover Ratio and a 53.3% decrease in stockout rate. Time series analysis revealed a long-term improvement trend in inventory efficiency with significant seasonal components. This research provides valuable insights for practitioners and academics on the critical role of demand forecasting in optimizing inventory management and the potential of the ARIMA method in improving companies' operational efficiency.