Background: The Modified Baecke Physical Activity Questionnaire (mBQ) is one of the most widely used questionnaires to assess physical activity in both healthy individuals and patients. Originally, the BQ has been developed and validated in English and its validation in different languages and cultures seems to be very important for obtaining reliable and valid data. Purpose: The purpose of this study is to translate and present the psychometric properties of the Greek version of mBQ in healthy people using translation protocols consistent with internationally recognised guidelines. Methods: For the cross-cultural adaptation, the back-translation procedure was utilised, comprising forward and backward translations by 4 bi-lingual translators. In pilot study the questionnaire was completed from 30 healthy subjects for linguistic validation and understanding. A further sample of 255 healthy volunteers (132 males, 123 females) aged 13–60 years old (Mean: 25.6, SD: 9.4) was included for the validity study. 100 of these subjects were requested to complete 2 questionnaires for concurrent validity; the mBQ as developed in its final Greek version and the Greek version of the International Physical Activity Questionnaire (IPAQ). Questionnaires were personally administered and completed via structured interviews by one physiotherapist. A second group of 155 volunteers (99 with normal activity and 56 athletes) was employed for the construct validity (known-group technique). For the reliability study a sample of 100 subjects (55 males, 45 females) aged 18-years old (Mean: 26.5, SD: 9.5) was included. Subjects were requested to complete the mBQ in two occasions, one week apart. For the statistical analysis Spearman rho correlation coefficient was used for the several correlations among questionnaires. Independent t-test was used for group differences. Intraclass correlation coefficient (ICC1,1), Standard Error of Measurement (SEM), Smallest Detectable Difference (SDD), Cronbach α, and evaluation of floor and ceiling values in proportion of minimum and maximum scores were all used for the reliability study using the Statistical Package for the Social Sciences (SPSS, version 20.0). Results: Content validity of the Greek version of the mBQ was achieved as all participants (including the ones in the pilot) found the questionnaire appropriate and comprehensible. Concurrent validity of mBQ with IPAQ demonstrated a significant moderate correlation (Spearman r = 0.43, p = 0.00). Differences among athletes and volunteers with normal activity were also significant (t = −6.93; p = 0.00) indicating the ability of mBQ to differentiate known-groups. Test–retest of mBQ was excellent with very small error between measurements (ICC = .84; SEM = 0.48; SDD = 16.7%). Internal consistency was also very high (Cronbach's α = 0.92) and similar with relevant studies. No specific ceiling or floor effect was evident. Conclusion(s): The Greek version of the mBQ has proven to be valid, reliable, comprehensible and acceptable for the Greek population tested. Implications: The Greek version of mBQ is a valid and reliable tool for the assessment of habitual physical activity and could be used with confidence from researchers and clinicians assessing Greek-speaking population both healthy and patients.