OBJECTIVES: A reliable and valid measure is essential to evaluating and enhancing patient medication adherence. There is no validated patient-reported medication adherence measure in Singapore. This study aimed to validate the 8-item Morisky Medication Adherence Scale (MMAS) in patients taking warfarin in Singapore. METHODS: A cross-sectional survey was conducted in a convenient sample of 174 patients taking warfarin at an anticoagulation clinic in Singapore in 2011. Socio-demographics and International Normalized Ratio (INR) values were obtained from patient interview and hospital databases. Respondents completed the MMAS in English or Chinese depending on their preference. The scale scores ranged from 0 to 8, with higher scores indicating better medication adherence. Reliability was assessed using Cronbach’s alpha. Criterion-related validity was examined by relating the MMAS score to warfarin refill rate. Construct validity was examined via factor analysis and hypothesis testing. RESULTS: The reliability of the MMAS was moderate (Cronbach’s alpha = 0.56). The scale scores were associated with warfarin refill rates (p = 0.02). Confirmatory factor analysis showed that the eight items loaded onto one factor (RMSEA = 0.048). Respondents with higher MMAS scores had a higher percentage of INRs within the therapeutic range during the past 2 weeks (p = 0.02), higher adherence to diet recommendations (p = 0.01), and less perceived difficulty in taking all medications (p <0.01). Respondents with higher scale scores were also more likely to take warfarin at the same time every day (p <0.001) and follow the prescribed dosage (p = 0.04). The hypotheses regarding the association between the scale score and the time within the therapeutic INR range, however, were not met. CONCLUSIONS: The 8-item MMAS had a good validity and moderate reliability in patients taking warfarin. Future research requires investigation of the scale’s reliability and psychometric properties in other patient populations and clinical settings.