Objectives Psychological inflexibility (PI) is a potentially important yet understudied factor in therapist training, as it may hinder engagement in learning activities central to psychotherapy education and is negatively associated with learner wellbeing. However, few instruments have been developed to assess PI in educational and training contexts, and evidence from psychotherapy training populations are limited. This study aimed to develop a new measure of PI intended for use in educational settings, and to evaluate its psychometric properties within the specific context of psychotherapy training. Method In Study One, an initial version of the scale was developed and tested with 195 mental health professionals who were currently or previously enrolled in psychotherapy continuing education. Preliminary psychometric evaluation included expert review, cognitive interviews, exploratory factor analysis, and Mokken scale analysis. Rasch methodology was then applied to assess dimensionality, item independence, response category ordering, targeting, invariance, and reliability. In Study Two, a new sample (n = 337) was used to validate the revised instrument through Rasch analysis. Results Results supported a unidimensional, 7-item scale demonstrating good model fit, local independence, ordered thresholds, invariance, and acceptable reliability. Additional analyses suggested a small but statistically significant negative association between age and psychological inflexibility (rho = −0.18), while no consistent differences were found across gender or profession. Conclusion The resulting instrument – the Karolinska Inner-Barriers Scale for Education (KIBS-Ed) – offers a brief and psychometrically sound measure of psychological inflexibility, suitable for use in educational and training settings. Our evaluation within two samples from the psychotherapy training population supports its application within therapist education. Despite providing only preliminary validation and requiring further evaluation with additional outcomes and instruments, the present study represent a rigorous and important first step