Background and Aim: Cardiovascular diseases are often associated with poor outcomes, including exercise intolerance. Endothelial dysfunction may contribute to impaired vascular homeostasis, thereby affecting the functional capacity of patients with cardiovascular disease. This study aimed to investigate the association between functional capacity and endothelial function in these patients. Methods: Nine patients diagnosed with cardiovascular disease who were admitted to an outpatient cardiac rehabilitation clinic were included in the study. Endothelial function was assessed through the flow-mediated dilatation (FMD) technique, which measured both absolute (mm) and percentage (%) changes in brachial artery diameter. Functional capacity was evaluated using the 6-minute walk distance (6MWD) test. The Shapiro–Wilk test was used to assess data normality, while Pearson’s correlation analysis and a linear regression model were applied to examine the relationship between functional capacity and endothelial function, with a significance level of 5%. Results: Significant positive correlations were found between 6MWD and FMD. The Pearson correlation coefficients were 0.80 (p = 0.01) for the absolute change (mm) in FMD and 0.81 (p = 0.01) for the percentage change in FMD. The linear regression model explained 58.8% of the variance in 6MWD (R² = 0.588, t = 12.385, p = 0.01), with the equation for 6MWD = 236.240 + (12.721 × FMD%). Conclusion: The findings suggest that functional capacity is closely associated with endothelial function in patients with cardiovascular disease. Specifically, the 6MWD increased as endothelial function improved.