The authors examined evaluation within supervision as a potential mediator in the relationship between counseling self-efficacy and working alliance among Turkish guidance and psychological counseling graduate students. Using convenience sampling, data were obtained from 173 (119 females and 54 male) Guidance and Psychological Counseling graduate program students. The Evaluation within Supervision Inventory, the Counseling Self-Efficacy Scale, and the Working Alliance Inventory-Short Form were used for data collection. The bootstrap resampling method was employed using Hayes’s SPSS PROCESS macro. Results of bivariate correlations showed that higher evaluation within supervision was associated with increased counseling self-efficacy and increased working alliances, as expected. The mediation model was found to be significant for the contribution of the evaluation within supervision. Based on the findings, providing effective evaluation within supervision is essential for counselors’ beliefs in their professional capabilities and for building a quality relationship between counselors and their clients. These findings offer useful suggestions for counselors, educators, and supervisors in higher education.