Literature assessing the use of long-acting injectable paliperidone palmitate in patients with bipolar I disorder is limited. The aim of this retrospective study was to determine the effectiveness of long-acting injectable paliperidone palmitate treatment on relapse and hospitalization in a real-world setting. Patients with bipolar I disorder aged 18-65 years, who were treated with paliperidone palmitate once-monthly (PP1M) for at least one year, were included. The rate of relapse, hospitalization, and length of hospital stay were collected. Safety outcomes included levels of prolactin, fasting blood sugar, total cholesterol, triglyceride, high density lipoprotein, and low density lipoprotein. The data of 36 patients who met the study criteria were evaluated. Number and length of hospitalizations, number of manic and mixed episodes significantly decreased after PP1M addition. When we compared the prolactin, fasting blood sugar, total cholesterol, triglyceride, high density lipoprotein, and low density lipoprotein levels as an indicator of the safety of treatment, there was no statistically significant change in these values before and after PP1M addition. Our findings suggested PP1M may be effective in reducing manic and mixed episodes. Limitations include a mirror image retrospective design and small sample size.