Background: A couples' psycho‐educational program called Oncofertility! Psycho‐Education and Couple Enrichment (O!PEACE) therapy was created and its effect when provided before cancer treatment was examined. Methods: This multicenter randomized controlled trial with nonmasking, parallel two‐group comparison enrolled women aged 20 to 39 years with early‐stage breast cancer and their partners. They were randomly assigned to receive O!PEACE (37 couples) or usual care (37 couples). Primary end points were cancer‐related posttraumatic stress symptoms, symptoms of depression, and anxiety. Secondary end points were stress‐coping strategies, resilience, and marital relationship. Results: Women receiving psycho‐educational therapy had significantly reduced Impact of Event Scale‐revised version for Japanese scores (p =.011, ηp2 = =.089). For patients with Impact of Event Scale‐revised version for Japanese scores at baseline ≥18.27, O!PEACE therapy improved these scores when compared with usual care (U = 172.80, p =.027, r = 0.258). A >5‐point reduction was present in 59.3% and 30% of women in the O!PEACE therapy and usual‐care groups, respectively. For partners, O!PEACE therapy significantly improved stress‐coping strategies (95% CI, −0.60 to –0.05; p =.018, ηp2 = =.074) and escape‐avoidance marital communication (95% CI, –0.33 to –0.08; p =.001, ηp2 =.136). O!PEACE therapy significantly improved the partners' support (95% CI, 0.10–0.50; p =.001, ηp2 =.127), the rate of receiving fertility preservation consultations, and knowledge levels. Conclusions: O!PEACE therapy before cancer treatment can improve posttraumatic stress symptoms, stress‐coping behavior, and marital relationships. Larger sample sizes and longer term follow‐up are required. Plain Language Summary: A psycho‐educational program, the Oncofertility! Psycho‐Education and Couple Enrichment (O!PEACE) therapy program was developed and evaluated for women diagnosed with breast cancer and their partners.A multicenter randomized controlled trial showed that the O!PEACE psycho‐educational therapy, with only two precancer treatment sessions, can reduce cancer‐related posttraumatic stress symptoms and improve oncofertility knowledge and marital relationships in young adult patients with breast cancer.The therapy could also improve stress‐coping strategies in marital communications with their partners. Couples may use O!PEACE psycho‐educational therapy to consider fertility preservation and improve their psychosocial aspects. A psycho‐educational program, the Oncofertility! Psycho‐Education and Couple Enrichment (O!PEACE) therapy program, was developed and evaluated for women diagnosed with breast cancer and their partners. O!PEACE therapy before cancer treatment can improve posttraumatic stress symptoms, stress‐coping behavior, and marital relationships. [ABSTRACT FROM AUTHOR]