Non-adherence to pharmaceutical treatment is one of the most common causes of uncontrolled hypertension. Non-adherence may be intentional or unintentional. In the case of intentional non-adherence, it is crucial to understand the reasons behind it. The literature increasingly addresses the issue of beliefs and concerns about medication, but studies on this subject performed in a Polish population of hypertensive patients are still lacking. The aim of the study was to assess the level of intentional non-adherence among patients with hypertension, and to determine the relationship between beliefs about medication and the level of intentional non-adherence to treatment in elderly patients with hypertension. The study included 300 patients (106 of whom were male, mean age (SD) 71.71 (8.12) years) with hypertension, treated at a hypertension clinic. The following instruments were used: the Intentional Non-Adherence Scale (INAS) for evaluating intentional non-adherence, and the Beliefs about Medicines Questionnaire (BMQ) for evaluating patients' beliefs and opinions regarding medication. Socio-demographic and clinical data were obtained from patients' medical records. The mean (SD) INAS score in the study was 47.28 (19.12). Patients were most concerned about the harm caused by medication, and least concerned about the necessity to take medication (mean score per item 3.49 vs. 2.14). Correlation analysis demonstrated weak correlations between BMQ and INAS: higher scores for necessity were associated with more intentional non-adherence (r = 0.174, p = 0.003), while higher scores for overuse, harm, and concerns were associated with less intentional non-adherence (respectively: r = -0.253, p < 0.001 vs. r = -0.336, p < 0.001 vs. r = -0.351, p < 0.001). In multiple-factor analysis, factors increasing the level of intentional non-adherence were elderly age (β = -0.352, p = 0.009), multimorbidity (β = -2.374, p = 0.035), and a higher BMQ concerns score (β = -1.376, p < 0.001), while being single was an independent predictor decreasing intentional non-adherence (β = 5.646, p = 0.013). The overall level of intentional non-adherence among patients with hypertension is moderate, but approximately one third of patients with hypertension demonstrate a high level of non-adherence. Independent determinants of intentional non-adherence include concerns, elderly age, multimorbidity, and being single.