Objective: We investigated the effectiveness of a self-help intervention named PER-ANTARA, which aims to improve adherence to diagnostic procedures among women with breast cancer (BC) symptoms to reduce the time to a definitive diagnosis.Methods: With a cluster randomized crossover design across four hospitals, PER-ANTARA and treatment as usual (TAU) or TAU only was provided at successive periods in a randomly determined order. The main outcome was the time between the first medical consultation and the definitive diagnosis. Secondary outcomes were BC knowledge, measured by the Breast Cancer Knowledge Test (BCKT); symptoms of anxiety and depression, measured by the Hospital Anxiety and Depression Scale (HADS); quality of life, measured by the World Health Organization Quality of Life-BREF (WHOQOL-BREF); and health status, measured by the EQ-5D-5L. A linear mixed model analysis was conducted to analyse the outcomes. Results: We recruited 132 women with BC symptoms from four hospitals; 67 participants were in the intervention group, and 65 participants were in the control group. PERANTARA reduced the time to definitive diagnosis by 13.3 days (M [SD]: 25.90 [23.20] in the intervention group vs 39.29 [35.10] in the control group; mean difference = −13.26, 95% CI = −24.51 to −2.00, P = .02). No significant difference was found between the groups in BC knowledge, symptoms of anxiety, depression, quality of life, or health status. Conclusions: PERANTARA reduced the time to definitive diagnosis among Indonesian women with BC symptoms. Psychoeducation may be an important addition to regular BC care to prevent undue delays in diagnostic procedures. K E Y W O R D S adherence, breast, cancer, cluster randomized controlled trial, diagnosis, health education,