Introduction: Cancers of the kidney, arising from either the renal parenchymal tissue or the renal pelvis, is among the 13 commonest types of malignancy globally, accounting for between 3% and 4% of all newly diagnosed cancers. Renal cell carcinoma (RCC) accounts for 85% of all malignant renal neoplasms. We present a rare case of an RCC directly extending into the renal pelvicalyceal system and with a thrombus within the ureter. Case Presentation: A 39-year-old woman presented with a long-standing history of worsening left flank pain, intermittent visible hematuria, and a fullness in the left flank. Apart from an ill-defined left flank mass with pain on palpation, there was nothing remarkable on clinical examination. Contrast-enhanced abdominal CT scan images showed a large, heterogeneously enhancing soft tissue mass arising from the lower pole of the left kidney. The collecting system and the left proximal ureter were poorly visualized. A tentative diagnosis of RCC, cT3aN0M0 was made. Intraoperatively, we identified a large, left lower pole renal mass, displacing the pedicle superiorly. In addition, we found a bulky, dilated proximal ureter. A decision was made intraoperatively to proceed with radical nephrectomy and ureterectomy. Conclusion: We report a rare case of RCC directly invading from the renal pelvis and down the ureter as a thrombus mass, with no microscopic individual tumor implants in the ureter wall, invasion of the renal vein, or invasion of adjacent organs. To our knowledge, only four such cases have been reported in English literature, and as a result, very few theories explaining renal pelvic invasion and direct growth down the ureter have been postulated. This highlights the significance of adding invasion of the pelvicalyceal system as part of the most recent, updated tumor node metastases classification. In future, consideration needs to be given to include the extension of RCC into the ureter and/or bladder.