Background: Marital infidelity is an interpersonal trauma that can lead to cognitive, emotional, and behavioral symptoms in the affected person. Objective: The goal of this study was to determine the effectiveness of emotion-focused couple therapy in insecure attachment styles in women and men affected by spousal infidelity. Methods: This study was quasi-experimental research with pre-test, post-test and follow-up with control group. The statistical population of this study included all men and women who were referred to counseling centers in Tehran in 2019-20 due to spousal infidelity. A total of 40 patients were randomly assigned to two groups: experimental and control, using a convenience sampling procedure. The intervention was not given to the control group. The Control group did not receive the intervention. Data were collected using an attachment styles questionnaire (Hazen & Shaver, 1987), and a multivariate covariance analysis test was used to eliminate the effect of pre-test using SPSS software. Results: The results demonstrated that emotion-focused couple therapy affected the scores of insecure attachment styles (P<0. 01). It was also revealed that the method of training was to enrich couples' relationships with an emotion-focused approach in the follow-up phase (P<0. 01). Conclusion: As evidenced by the results of this study, it can be stated that emotion-focused couple therapy is effective in women and men affected by spousal infidelity,therefore, it can be successfully used as a therapeutic or educational method in couple therapy programs.