The data about continuing trastuzumab treatment beyond progression in HER2-positive breast cancer are lacking in Iraq. This study aimed for investigating the use of trastuzumab in Iraqi women with metastatic HER2-positive breast cancer. A single-arm non-comparative observational prospective study was conducted in Oncology Teaching Hospital between 2013 and 2019. The study included metastatic HER2-positive breast cancer progressing after first-line chemotherapy plus trastuzumab. Only 16(27%) cases had tumor progression. Median time to progression was 16 months (95% CI:11.0–20.9). 55(93%) patients had a progression-free survival of ≥ 6 months, whereas 32(54%) had a progression-free survival of ≥ 12 months. The continuing trastuzumab beyond progression has clinical benefits in HER2-positive metastatic breast cancer.