Approximately a quarter to a half of all people fail to take their medication regimen as prescribed (i.e. non-adherence). Conscientiousness, from the five-factor model of personality, has been positively linked to adherence to medications in several recent studies.This study aimed to systematically estimate the strength and variability of this association across multiple published articles and to identify moderators of this relationship.A literature search identified 16 studies (N = 3,476) that met the study eligibility criteria. Estimates of effect sizes (r) obtained in these studies were meta-analysed.Overall, a higher level of conscientiousness was associated with better medication adherence (r = 0.15; 95 % CI, 0.09, 0.21). Associations were significantly stronger in younger samples (r = 0.26, 95 % CI, 0.17, 0.34; k = 7).The small association between conscientiousness and medication adherence may have clinical significance in contexts where small differences in adherence result in clinically important effects.