Poor adherence is a leading cause of apparent resistance to antihypertensive treatment. Recent empirical research has investigated predictors of adherence for primary care patients who are apparently resistant to treatment; however, questions remain regarding the variability in adherence behaviour among this group. This study aimed to investigate factors that may elucidate medication adherence among patients with apparent treatment-resistant hypertension (aTRH) using qualitative methods. Fourteen semi-structured interviews were conducted with patients undergoing treatment for aTRH in primary care in the West of Ireland. Patients who self-reported both high and low adherence in a previous quantitative study were purposively sampled. Data were analysed using thematic analysis. A public and patient involvement research group were active partners in developing the study protocol and interview topic guide. Three major themes were identified: beliefs about treatment, habits and routine, and health and health systems. High adherers reported favourable beliefs about antihypertensive treatment that had been validated by experience with taking the treatment over time, described strong medication-taking habits and stable routines, and positive relations with their GP. Low adherers expressed less coherence in their beliefs and used less effective strategies to support their medication-taking in daily life. The current findings are consistent with qualitative studies of adherence in other chronic conditions. Results reflect the difficulty for healthcare practitioners in identifying adherent versus non-adherent patients via conversation, and highlight the importance of accurate adherence assessment. Inception studies may provide an opportunity to better understand adherence behaviour across the illness trajectory.