Despite the effectiveness of drug therapy in diabetes management high rates of poor adherence persist. The purpose of this study was to identify potentially modifiable patient disease and medication beliefs associated with poor medication adherence among people with diabetes. A cohort of patients with diabetes was recruited from an urban primary-care clinic in New York City. Patients were interviewed in English or Spanish about: disease beliefs, medication beliefs, regimen complexity, diabetes knowledge, depression, self-efficacy, and medication adherence (Morisky scale). Logistic regression was used to identify multivariate predictors of poor medication adherence (Morisky > 1). Patients (n = 151) had diabetes for an average of 13 years with a mean HgA1C of 7.6 (SD 1.7). One-in-four (28%) were poor adherers to their diabetes medicines. In multivariate analyses, predictors of poor medication adherence were: believing you have diabetes only when your sugar is high (OR = 7.4;2–27.2), saying there was no need to take medicine when the glucose was normal (OR = 3.5;0.9–13.7), worrying about side-effects of diabetes medicines (OR = 3.3;1.3–8.7), lack of self-confidence in controlling diabetes (OR = 2.8;1.1–7.1), and feeling medicines are hard to take (OR = 14.0;4.4–44.6). Disease and medication beliefs inconsistent with a chronic disease model of diabetes were significant predictors of poor medication adherence. These suboptimal beliefs are potentially modifiable and are logical targets for educational interventions to improve diabetes self-management.