Wilms' tumour (nephroblastoma) is the most common renal tumour in children. Wilms' tumour in adults is extremely rare and has a poorer prognosis than paediatric Wilms' tumour. It is difficult to differentiate adult Wilms' tumour from renal cell carcinoma based on radiological findings alone. The diagnosis in adults is often serendipitous following nephrectomy for presumed renal cell carcinoma. Because of the paucity of literature, there are no standard protocols for the management of adult Wilms' tumour, and therefore, it is managed as per paediatric Wilms' tumour. Herein, we report the case of adult Wilms' tumour in a 43-year-old man, which was diagnosed unexpectedly following nephrectomy for presumed renal cell carcinoma.