The incidence of pregnancy-associated breast cancer (PABC) is increasing nowadays, and its diagnosis and treatment remain complicated due to the consideration of the fetus. The available data on PABC are primarily derived from case reports since there are ethical restrictions on conducting randomized clinical trials. In the present work, we reported a case of the human epidermal growth factor receptor 2 (HER2)-positive PABC and described the diagnosis and treatment for such type of breast cancer. A 27-year-old patient was admitted to our hospital with the complaints of right breast mass for 3 days, and she was a first-time pregnant woman with a single live intrauterine fetus at 26 + 3 weeks of gestation. Physical examination of the right breast revealed a palpable and hard mass with obscure boundaries (5.0 cm × 4.0 cm) in the upper outer quadrant. Significant axillary lymph nodes (2.0 cm) were also present. PABC. To protect the fetus, breast ultrasonography was used to test her breast mass, a core needle biopsy was adopted to confirm the diagnosis, and abdominal ultrasound and chest X-ray were used to evaluate the metastasis. The patient was scheduled for neoadjuvant therapy using bi-weekly pirarubicin in combination with cyclophosphamide (AC) without anti-HER2 therapy for consideration of the fetus's safety. After 4 cycles of AC, the patient delivered a healthy male infant. After the delivery, all the treatments were carried out according to the standard recommendation for HER2 + breast cancer as non-pregnant patients. After the surgery, the disease-free survival for the patient was 12 months until brain metastasis was diagnosed. She was still undergoing second-line anti-HER2 therapy and currently in a stable situation. Besides, the child was also healthy so far. The methods for the diagnosis and treatment of PABC that result in teratogenesis should be avoided to protect the fetus. Mammogram and chest X-ray were safe approaches for the fetus. Moreover, chemotherapy-based on pirarubicin in combination with cyclophosphamide had no risk to the fetus.