Primary mucinous adenocarcinoma of the renal pelvis is an extremely rare tumor with only a handful of cases reported to date. Clinical and radiological features are not specific, and hence, histopathological examination holds the key for definitive diagnosis. This tumor has mainly been described in the elderly population, with less than five cases reported in individuals aged <35 years. Here, we report a case of primary mucinous adenocarcinoma of the renal pelvis in a young male. A 31-year-old male presented with a history of right-sided flank pain for the past year. On examination, he had right-sided costovertebral tenderness. Computed tomography (CT) scan revealed the presence of a hyperdense mass lesion in the right renal pelvis with severe hydronephrosis and cortical thinning. Because of the non-functioning status, right nephrectomy was performed. To our surprise, histopathology showed the presence of mucinous adenocarcinoma of the renal pelvis with carcinoma in situ of the ureter. This case describes a rare presentation of primary mucinous adenocarcinoma of the renal pelvis, and highlights the importance of histopathological examination in reaching the correct diagnosis.