Introduction and aim: In recent years, numerous health-related apps have appeared on the market, and assessing their quality has become crucial. A very popular tool worldwide, created especially for this purpose, is the Mobile Application Rating Scale (MARS). However, there are no similar tools in Spanish. Therefore, the aim of this paper is to adapt MARS to the Spanish language and validate the resultant version. Method: The design consists of three processes: cross-cultural adaptation, translation, and metric evaluation. 46 mobile applications, 23 of which were for Apple telephones and 23 of which were for telephones running Android, were included in the study. The main objective of these applications was to promote physical activity. The internal factor structure and reliability of MARS were examined. Results: No major differences were observed in the two Spanish translations, which were carried out independently. The blind back-translation, reviewed by the original author of MARS, suggested minor edits. Discrimination indices (item-scale correlation) obtained appropriate results for both raters. The reliability of the scores was found to be appropriate both in terms of internal consistency (alpha > 0.77), temporal stability (r > 0.72), and inter-rater reliability (IC > 0.76). The correlations between the subscales have shown high values with range between 0.47 and 0.83. Conclusion: The Spanish version of MARS was shown to have appropriate metric properties to assess the quality of health apps.