This study aims to determine the effect of Customer Relationship Management (CRM) on customer loyalty at PT Bank Mandiri (Persero) Tbk Padangsidimpuan Branch. The research method used is quantitative, with a data collection approach through observation, questionnaires, interviews, and documentation studies. Data analysis was carried out using simple linear regression techniques, hypothesis testing (t test), and the coefficient of determination (R²) test. The results showed that the regression coefficient value was +0.509, which indicated a positive relationship between CRM and customer loyalty. Hypothesis test analysis produces at value of +6.364 with a significance level of 0.000, which is smaller than 0.05. This shows that CRM has a positive and significant effect on customer loyalty partially. Based on the coefficient of determination analysis, the R² value of 43.9% indicates that CRM is able to explain 43.9% of customer loyalty variability, while the remaining 56.1% is influenced by other factors not discussed in this study. Thus, this study concludes that Customer Relationship Management has a significant influence on customer loyalty at PT Bank Mandiri (Persero) Tbk Padangsidimpuan Branch. These results strengthen the hypothesis that CRM is an important factor in building and increasing customer loyalty.