The growing body of research on mindfulness has led to the development of a wide range of self-report instruments designed to assess the construct. However, conceptual and psychometric issues have arisen, partly as a result of inconsistent definitions and operationalizations, but also from challenges in scale development and validation. This systematic review synthesizes and evaluates the psychometric qualities of 21 self-report mindfulness scales intended for adult populations and tested in their English versions. The scales were evaluated in reference to nine psychometric quality domains, each rated on a 4-point scale. The findings show that while responsiveness and internal consistency were typically adequate across measures, other psychometric characteristics, including discriminant validity, test-retest reliability, and floor/ceiling effects, were either inadequately reported or performed inconsistently. In addition, content validity varied widely, and most measures did not fully align with a newly refined definition of mindfulness. Notably, none of the reviewed scales measured mindfulness as the co-occurrence of present-moment awareness and attitudinal aspects within individual items, thus limiting their ability to assess mindfulness as an integrated construct. This review underscores persistent conceptual and psychometric limitations and calls for the development of a new self-report measure that effectively balances theoretical comprehensiveness with methodological robustness. Future efforts should aim to integrate the richness of mindfulness theory with the demands of rigorous psychometric evaluation, thereby strengthening treatment evaluation and therapeutic monitoring in the context of mindfulness-based interventions and programs.