Epistemic trust (ET), the ability to evaluate and accept knowledge from others, has gained increasing recognition as a core process in psychotherapy. Disruptions in this capacity, often rooted in early adverse experiences, can reduce patients' openness to therapeutic input and limit treatment gains. This systematic review synthesizes 17 empirical studies (2014-2024) that investigated epistemic trust across diverse psychotherapies, including mentalization-based treatment, psychodynamic therapy, and multimodal rehabilitation. Findings highlight epistemic trust as both a mechanism of therapeutic change and an outcome of successful intervention, closely tied to alliance formation, engagement, and symptom improvement. Measurement approaches varied widely: self-report tools such as the ETMCQ and observational systems such as the ETRS demonstrated promising validity, yet methodological heterogeneity and limited cross-study comparability remain challenges. Importantly, current observational methods are still in early stages and require refinement to capture the dynamic, interactional, and culturally embedded nature of ET in psychotherapy. Notably, most studies relied on WEIRD samples, with underrepresentation of children, older adults, and non-Western populations. ET is emerging as a clinically actionable and transdiagnostic construct that underpins alliance, engagement, and outcomes across psychotherapies. Future research should adopt longitudinal, culturally sensitive designs to clarify developmental trajectories of ET and its role in sustaining therapeutic change.