Non-adherence to antihypertensive medication is the most important cause of uncontrolled blood pressure and is influenced by multiple interrelating factors. Understanding the complexity of medication non-adherence and its associated factors is important to determine intervention strategies. Therefore, a systematic review was performed aimed to identify factors associated with antihypertensive medication non-adherence. Different databases were searched for observational studies reporting on factors associated with non-adherence to antihypertensive medication. Titles, abstracts and full texts were reviewed by three researchers. Subsequently, the methodological quality of each study was assessed. Factors that were extracted from the included studies were categorised as factors with consistent or inconsistent evidence to put their potential importance into perspective. Forty-four studies were included. Higher co-payment, side effects and a poor patient-provider relationship were identified as factors with consistent evidence since consistent significant relationships were found for these factors whenever studied. The relationships between non-adherence and multiple other factors were inconsistent among the reviewed studies. However, some of these factors deserve some consideration. Since multiple potentially relevant factors were identified, patient-tailored interventions focussing on identifying and addressing patients' specific barriers to adherence are needed. Further research should clarify the influence of inconsistent factors on adherence and their potential to be addressed in interventions.