Purpose: This study aimed to determine the effectiveness of psychological workplace counseling for South Korean employees.Methods: Psychological counseling sessions were conducted for 93 employees from 11 companies that signed a contract for psychological counseling services with the Workplace Mental Health Institute, Kangbuk Samsung Hospital. Each 50-minute session was tailored to the individual needs of the clients. The Clinical Outcomes in Routine Evaluation-Outcome Measure (CORE-OM) counseling effectiveness assessment scale was administered before and after the counseling. A paired t-test was used to compare the mean scores before and after the sessions, and an analysis of variance (ANOVA) was performed, followed by a Scheffe post hoc analysis.Results: In the overall sample, the mean CORE-OM scores before and after counseling showed a significant decrease from 1.343 to 0.883, with improvements observed in all four sub-domains. ANOVA showed that the group with more than 11 sessions showed greater change in scores than the group with four sessions or fewer. The reliable change index scores showed that 38% of the employees demonstrated a reliable change, and the groups with eight or more sessions exhibited greater change than the group with fewer than eight sessions.Conclusion: Among South Korean employees who received workplace counseling services, both the mean total score of the CORE-OM and the mean scores of the four sub-domains decreased after counseling. In addition, the effectiveness of counseling significantly increased as the total number of sessions increased.