The 8-item Morisky Medication Adherence Scale (MMAS-8) is reliable and valid in patients with hypertension, but to our knowledge validity has not been established for patients with asthma. The aim of the study was to determine the criterion validity of the MMAS-8 in patients with asthma. In the cross-sectional study patients older than 12 year were recruited when dispensed asthma medications in community pharmacies. Criterion validity of the scale was assessed through associations with asthma control and quality of life. Asthma control was assessed by the Asthma Control Test (ACT) and quality of life was evaluated by the Saint George Respiratory Questionnaire (SGRQ). A total of 208 patients (mean age 56 years, 59% female) were included in the study. Almost all patients were prescribed inhaled corticosteroids (96%). Asthma was not controlled in 37% of the patients and 22% experienced at least one exacerbation requiring emergency room visit, hospitalization or treatment with oral corticosteroid therapy in the previous year. The 8-item MMAS was significantly associated with asthma control and quality of life. Patients who scored 8 points, <8 to >6 points and ≤6 points on the scale were considered to have high, medium and low adherence, respectively. High, medium and low adherence was found in 53%, 23% and 24% of the patients, respectively. As adherence improved from low to medium or from medium to high, the odds of asthma control increased by 1.7 times (OR 1.65, p = 0.027). Patients with high and medium adherence had SGRQ scores that were 6.1 and 5.3 points lower, respectively, compared with patients with low adherence. The MMAS-8 was found to be valid for assessing medication adherence and predicting health outcomes in patients with asthma.