So far there has been no instrument with proven validity and reliability for patient-reported outcome assessment of rheumatoid arthritis in Estonia. The aim of our study was to adapt the Health Assessment Questionnaire’s Disability Index (HAQ-DI) for Estonia and to assess its psychometric properties. The linguistic validation included the phases of translation and back translation and testing on patients. Reproducibility and construct validity were estimated using a sample of 50 rheumatoid arthritis patients. The participants were administered the HAQ-DI and the comparator instruments at two visits; disease activity and radiological stage were also assessed. The participants were asked to comment on the questionnaires. The HAQ-DI was easily translatable into Estonian. It was shown to have high test-retest reliability, internal consistency and the ability to discriminate between groups defined by patient-perceived severity. Comparison with the comparator measure demonstrated predicted convergent and divergent validities. Still, the comments of the participants revealed some issues disregarded during the preceding adaptation process. The Estonian HAQ-DI can be recommended for inclusion in clinical studies and trials involving patients with rheumatoid arthritis; some aspects of the adaptation process are highlighted for further discussion.