The purpose of this study was to develop and validate the Patient-Reported Outcomes instrument for Thai patients with type 2 diabetes mellitus (PRO-DM-Thai) and to test its validity and reliability for use in Thai patients. The study consists of 3 steps as follows: 1) content development of the instrument by using literature review and in-depth interviews of provider and patients and testing content validity by experts’ assessment and content validity index (CVI); 2) construct validity and reliability testing by conducting a cross – sectional descriptive study on 500 participants during May to June, 2011, and performing confirmatory factor analysis and Cronbach’s alpha coefficient (α); and 3) criterion-related validity testing by a cross – sectional analytic study with 200 participants during September to November, 2011, along with Spearman’s Rank Correlation, ANOVA, Multiple linear regression, and Discriminant analysis. The study results showed the PRO-DM-Thai passed all of the validity tests. It consists of 7 dimensions, 44 items. These include Physical function, Symptoms, Psychological well-being, Self - care management, Social well-being, Global judgments of health, and Satisfaction with care and flexibility of treatment. The CVI of the instrument at the item-level (I-CVI) were between 0.83 to 1.00 and at the scale-level average agreement (S-CVI/Ave) was 0.98. All dimension models had overall fit with empirical data while the hypothesized model of PRO-DM-Thai demonstrated good fit (χ2=5.23, df=6, p>0.05, AGFI=0.986, RMSEA=0.000). For the internal consistency reliability, the Cronbach’s alpha coefficient (α) for the total scale was 0.91 and for the subscales was during 0.72 -0.90. The abilities to discriminate groups of patients with different level of disease control and between patients with and without complications by the total score of outcomes. In summary the instrument indicate satisfactory levels of validity and reliability, suitable for use in Thai diabetic patients.