<p>Authenticity, defined as the knowledge of one’s true self, living in accordance with one’s values and beliefs, and resistance to the influence of others, has gained increasing prominence within the realm of positive psychology. This paper presents the culmination of efforts to develop a Polish version of the Authenticity Scale (AS). The AS is a self-report instrument designed to examine three dimensions of authenticity: authentic living, accepting external influence, and self-alienation. To validate the psychometric properties of the Polish AS, a series of four studies were conducted, encompassing a total number of 1546 participants. The findings demonstrate the instrument’s high internal consistency (with Cronbach’s α ranging from .77 to .87), confirm the three-dimensional structure of authenticity (via confirmatory factor analysis), and provide evidence for measurement invariance across gender groups and under various testing conditions. Theoretical validity was established by investigating the relationships between the authenticity dimensions and a range of variables, including components of the PERMA model, psychological well-being, meaning in life, self-esteem, and personality traits. The observed associations align with theoretical expectations and corroborate the adaptive nature of authentic living and the maladaptive tendencies of accepting external influence and self-alienation. The obtained results collectively support that the psychometric properties of the Polish adaptation of the Authenticity Scale are comparable to those of the original version. It can therefore be concluded that the Polish adaptation of the AS is a reliable tool for measuring authenticity, which—once normalisation research has been conducted or local norms calculated—can be useful in the work of clinicians, psychological counsellors, and researchers.</p>