The Juvenile Arthritis Multidimensional Assessment Report (JAMAR) is a new parent/patient-reported outcome measure that enables a thorough assessment of the disease status in children with juvenile idiopathic arthritis (JIA). We report the results of the cross-cultural adaptation and validation of the parent and patient versions of the JAMAR in the Greek language. The reading comprehension of the questionnaire was tested in 10 JIA parents and patients. Each participating centre was asked to collect demographics, clinical data, and the JAMAR from 100 consecutive JIA patients or all consecutive patients seen in a 6-month period and to administer the JAMAR to 100 healthy children and their parents. The statistical validation phase explored descriptive statistics and the psychometric issues of the JAMAR: the three Likert assumptions, floor/ceiling effects, internal consistency, Cronbach's alpha, interscale correlations, test-retest reliability, and construct validity (convergent and discriminant validity). The Greek JAMAR was fully cross-culturally adapted with two forward and three backward translations. A total of 272 JIA patients (5.9% systemic, 57.7% oligoarticular, 21.3% RF negative poly-arthritis, 15.1% other categories), and 100 healthy children were enrolled in all centres. The JAMAR components discriminated well-healthy subjects from JIA patients; notably, there was no significant difference between healthy subjects and their affected peers in psychosocial quality of life and school-related items. All JAMAR components revealed good psychometric performances. In conclusion, the Greek version of the JAMAR is a valid tool for the assessment of children with JIA and is suitable for use both in routine clinical practice and in clinical research.