BACKGROUND Primary renal synovial sarcoma (PRSS) is an extremely rare tumor with a poor prognosis. Its imaging and immunohistochemical characteristics may overlap with other renal tumors, which renders its early diagnosis in a dilemma. The diagnosis of primary renal synovial sarcoma requires histopathology and the confirmation of SYT-SSX gene fusion using molecular techniques. Cases of primary renal synovial sarcoma have been previously reported in the literature. However, to our knowledge, primary renal allograft synovial sarcoma was never described. CASE SUMMARY A 43-year-old male patient who underwent kidney transplantation 9 months ago came to our hospital for regular follow-up. Traditional ultrasonography revealed multiple hypo-echo neoplasms in the renal allograft. Contrast-enhanced computed tomography (CECT) showed slightly hyper-density masses with slow homogeneous enhancement. Ultrasound-guided biopsy was conducted for accurate pathological diagnosis. The neoplasms were diagnosed as synovial sarcoma by pathological, immunohistochemical, and genetic analyses. Positron emission tomography/CT showed no evidence of metastasis. At approximately one week post biopsy, contrast-enhanced ultrasound was conducted to eliminate active hemorrhage. One month later, CECT showed that the biggest neoplasm grew from 3.3 cm to 5.7 cm in diameter. Parametric imaging was conducted with SonoLiver CAP to conduct further quantitative analysis, which showed that the enhancement pattern was heterogeneous hyper-vascular enhancement. Radical surgical resection of the whole renal allograft and ureter was conducted without additional adjuvant chemotherapy or external radiotherapy. Anlotinib was chosen for targeted therapy with a good response. CONCLUSION We propose multimodality imaging for accurate diagnosis of renal allograft synovial sarcoma especially when it is formed by spindle-shaped cells.