Background Primary retroperitoneal serous adenocarcinoma (PRSA) is an extremely uncommon malignancy exclusively reported in females. Due to the rarity of the disease, it is difficult to establish a standardized treatment. Case presentation We describe a unique case of PRSA in a 71-year-old male who presented with right-sided lower back pain and numbness. Magnetic resonance imaging identified a mass invading the adjacent psoas muscle and twelfth rib. Tissue biopsy confirmed poorly differentiated PRSA. Patient was initially treated with neoadjuvant carboplatin and paclitaxel chemotherapy regimen. This resulted in complete radiological resolution of the tumor. However, 12 weeks later, rapid recurrence was noted on follow-up CT scan. The patient was then treated with external radiotherapy with concurrent nivolumab, an anti-PD-1 antibody. The patient displayed a positive response to treatment with reduction in primary tumor and metastases and had a sustained disease control. Conclusion Treatment with radiotherapy in combination with anti-PD-1 antibody could be an effective modality of management for PRSA. Electronic supplementary material The online version of this article (10.1186/s12885-019-5934-4) contains supplementary material, which is available to authorized users.