Objective To overcome the limited effectiveness of standard case management services, the Seoul Intensive Case Management pro-gram (S-ICM) for patients with serious mental illnesses was introduced in 2017. This study aimed to evaluate its effectiveness in reduc-ing the length of hospital stay. Methods Monitoring data from April 2019 to March 2020 were retrieved from the Seoul Mental Health Welfare Center. A total of 759 participants with serious mental illnesses were included. The average length of admission per month was compared between the pre-ICM (previous year) and during-ICM periods. For post-ICM observation subgroup, average length of admission per month was compared between pre-ICM, during-ICM, and post-ICM periods. To determine the relative contributions of risk factors for during-ICM and post-ICM admission, multivariate logistic regression analyses were performed. Results The average admission stay for pre-ICM period was significantly longer than that for during-ICM period (1.47 vs. 0.26 days). Among the predictors for during-ICM admission, pre-ICM psychiatric admission was the most important risk factor, followed by medical aid beneficiary and suicidal behavior. In the subgroup analysis of the post-ICM observation period, the pre-ICM, during-ICM, and post-ICM average admission stays were 1.45, 0.29, and 0.57 days/month, respectively. There was a significant difference in the average length of stay between the pre-ICM and during-ICM periods and between the pre-ICM and post-ICM periods. Post-ICM admission risks includ-ed pre-ICM admission, S-ICM duration <3 months, and chronic unstable symptoms. Conclusion The results suggest that the S-ICM effectively reduces psychiatric hospitalization duration, at least over a short-term period.