Background Multiple factors affect treatment adherence in individuals with cardiovascular disease. However, information on the relationships among treatment adherence, family functioning, and self-care agency in these patients and their families is limited. Purpose This study was developed to determine the relationships among treatment adherence, family functioning, self-care agency, and sociodemographic variables in patients with cardiovascular disease. Self-care agency, as defined by Orem, is the dynamic process patients use to engage in their own healthcare that involves discerning and addressing factors that allow their making decisions that improve self-care abilities. Methods This cross-sectional, observational–analytical study enrolled 151 adult patients with cardiovascular diseases who had undergone pharmacological and nonpharmacological treatments and 108 family members of these patients who had consented to participate. Measurements were performed using the “Questionnaire for measuring treatment adherence in patients with cardiovascular disease,” the “Family Functioning Assessment Scale,” and the “Self-care Agency Scale.” Results Of the 151 patients, 119 (78.8%) were assessed as having a low risk of nonadherence, 60 (39.7%) as having low family functioning, and 131 (86.8%) as having high self-care agency. Treatment adherence and self-care agency showed a moderate and significant correlation (r = .66, p < .001). Similarly, treatment adherence and family functioning showed a low but significant correlation (r = .35, p < .001). Moreover, significant multivariate associations were found among the variables of interest. Patients with a low risk of nonadherence were found to be more likely to have a secondary or postsecondary education, not to have vision or hearing problems, and to have a contributory affiliation mode with the health system or private health insurance. In addition, participants with moderate or high levels of family functioning were less likely to be workers or to not have hearing or vision problems. Finally, significant differences were noted between patients with low self-care agency and those with high self-care agency in terms of kinship relationship with family members and affiliation mode with the health system. Conclusions/Implications for Practice The results of this research help clarify the issue of treatment adherence in patients with cardiovascular disease. Although family functioning and self-care agency were found to be low to moderately correlated with treatment adherence, relevant information regarding these variables and sociodemographic variables is presented in this study. Nurses may use these results as a reference to design nursing care plans and interventions to address the conditions of their patients more appropriately.