This study aims to validate the Greek version of the 54-item Reflective Functioning Questionnaire (RFQ), a measure designed to assess an individual’s capacity for understanding themselves and others based on internal mental states. This capacity, also known as Reflective Functioning (RF) or mentalizing, is believed to play a significant role in both typical and atypical development. The validation process examined the factor structure of the RFQ and its relationship with a variety of psychosocial and clinical constructs that have theoretical and empirical links to RF. Additionally, this research investigated the factor structure’s invariance across gender and age groups to determine the robustness of the instrument. A unique contribution of this work lies in examining the application of the RFQ to attachment classifications through the use of cluster analysis. The sample consisted of 875 Greek adults from the general community with a mean age of 28.5 and a median age of 22. Participants completed the Greek RFQ along with a series of self-report questionnaires assessing psychosocial constructs, including attachment, epistemic trust, emotion regulation, and psychological mindedness, as well as clinical variables such as anxiety, depression, and borderline personality traits. Our findings suggest that a shorter, 31-item version of the questionnaire provides a robust three-factor structure across a non-clinical Greek adult population. The three identified subscales are (a) excessive certainty, (b) interest/curiosity, and (c) uncertainty/confusion, all demonstrating satisfactory reliability and construct validity. The uncertainty subscale was found to be associated with insecure attachment styles, epistemic mistrust and credulity, emotional suppression, and low psychological mindedness. In contrast, the certainty and curiosity subscales were linked to secure attachment, epistemic trust, emotion reappraisal, and psychological mindedness. Uncertainty was further shown to differ significantly across probable clinical and non-clinical groups, as distinguished by cut-off scores for anxiety, depression, and borderline personality disorder (BPD). However, the certainty and interest/curiosity subscales only varied between the two BPD groups. Our results provide the first evidence supporting the use of a 31-item version of the RFQ with three validated subscales to reliably assess reflective functioning in the Greek population, demonstrating stronger psychometric properties compared to other RFQ versions reported in previous studies. Findings suggest that impaired mentalizing capacity, as measured by the RFQ, is linked to insecure attachment, epistemic mistrust and credulity, poor emotion regulation, and low psychological mindedness, and potentially plays a role in adult mental health symptoms.