Medication adherence is very important for the effective treatment or control of various health problems, including chronic disease like diabetes mellitus (DM). However, medication non-adherence among diabetic patients on follow-up treatment is still a global health problem. This study aimed to identify factors associated with medication adherence and to determine methods on how it could be improved. A cross-sectional study was conducted on medication adherence among Malays, Iban and Melanau ethnic groups in Kota Samarahan and Sarikei, Sarawak using the Health Belief Model framework. Interviews with questionnaires, which were tested for its validity and reliability using the Cronbach’s Alpha, were conducted to collect data on the respondent’s socio-demographic and economic characteristics, and health beliefs of 442 respondents. Data was analyzed using SPSS version 17.0 for frequency distribution, measures of central tendencies, significance testing and logistic regression. The medication adherence rates were low in terms of all the treatment indicators such as amount of medication (31.7%), frequency (38.9%), duration (26%), and follow-up treatment (24.2%). The respondent’s socio-economic and economic characteristics have statistically significant association with medication adherence. The respondents adhered towards medication because they believed in its benefits. They also took their medication because they believed in the severity of DM and their susceptibility to its serious complications. The cues to action (medication taking) such as worrying about their socio-economic well-being, effectiveness of medication, and health campaign on diabetic control have influenced medication adherence. However, forgetfulness, distance of clinic, and costs of transport have caused medication non-adherence. The respondent's health beliefs in the benefits of taking medication, perceived severity and susceptibility to DM and its serious complications have contributed towards medication adherence. Their concerns about the socio-economic well-being, effectiveness of medication, and health campaign on diabetic control were positive cues to medication taking behavior. Therefore, modifying the respondent's related health beliefs and reinforcing the positive cues to actions are the relevant intervention strategies that could be used in improving medication adherence among diabetic patients.