Indoleamine 2,3‐dioxygenase 1 (IDO1) is a key enzyme associated with immunomodulation through its regulation of the tryptophan‐kynurenine (Kyn) pathway in advanced cancers, including metastatic renal cell carcinoma (mRCC). However, the failure of IDO1 inhibitors when used in combination with immune checkpoint inhibitors (ICIs), as observed in clinical trials, raises a number of questions. This study aimed to investigate the association of tryptophan 2,3‐dioxygenase (TDO) and IDO1 with cancer development and resistance to immunotherapy in patients with RCC. In our analysis of RCC tissue samples, tissue Kyn levels were elevated in advanced‐stage RCC and correlated well with TDO expression levels in RCC tumor cells. In patients with mRCC, TDO rather than IDO1 was expressed in RCC tumor cells, showing a strong association with Kyn expression. Furthermore, immunohistochemical staining of TDO was strongly associated with the staining intensity of forkhead box P3, as well as ICI therapy response and survival in patients with mRCC. Our study is the first to show that TDO expression in tumor tissues is associated with progression and survival, confirming its potential as a predictive biomarker of primary resistance to immunotherapy in patients with mRCC. Our findings suggest that strategies aimed at inhibiting TDO, rather than IDO1, in combination with ICI therapy may aid in the control of mRCC progression. This study aimed to investigate the association of tryptophan 2,3‐dioxygenase (TDO) and indoleamine 2,3‐dioxygenase 1 (IDO1) with cancer development and resistance to immunotherapy in renal cell carcinoma (RCC) patients. Our results suggest that TDO expression in tumor cells is associated with kynurenine accumulation, progression, and survival, confirming its potential as a predictive biomarker of primary resistance to immunotherapy in RCC patients. We believe that our study makes a significant contribution to the literature because, to the best of our knowledge, it is the first to show that the accumulation of kynurenine induced by TDO in tumor tissues is associated with progression and survival. Further, we believe that this paper will be of interest to the readership of your journal because our results strongly recommend the testing of specific TDO inhibitors or dual inhibitors of IDO1 and TDO with ICI treatment in future preclinical and clinical trials focusing on mRCC.