 Background, Mucinus adenocarcinoma of the renal pelvis is a very uncommon tumor, most often secondary to chronic infection and long-standing irritation due to conditions such as urolithiasis. Case presentation, herein we report our experience with an old age male that presented with flank pain. Mucinus adenocarcinoma in situ has been discovered incidentally in his nefrectomy specimen. Conclusions, Mucinus adenocarcinoma in situ is extremely rare and to the best of our knowledge only two cases have been reported in the English literature so far. This article was authored by Bitta Jermazeta, Abdelaziz Kezri, and Rahil Jidi.