Renal cell carcinoma (RCC) is a common urologic tumor and the third leading cause of death among urological tumors. Recent studies demonstrate that RCC tumors are more heavily infiltrated by lymphocytes than other cancers. However, the exact roles played by CD4 + T cells in RCC proliferation remain unknown. In this study, we cocultured RCC cells with CD4 + T cells. Stable knockdown of YBX1 in RCC cells was constructed. The effects of CD4 + T cells, TGFβ1 and YBX1 on RCC cells were investigated using cell viability assays. In situ RCC nude mouse model was used to observe the tumor growth. The potential mechanisms of CD4 + T cells and YBX1 in RCC cells proliferation were explored by qRT-PCR and western blot. Expression of CD4, Foxp3 and TGFβ1 in RCC were quantified by immunohistochemical staining. The results indicated that CD4, Foxp3 and TGFβ1 were significantly up-regulated in RCC tissues. Human clinical sample and in vitro cell lines studies showed that RCC cells had better capacity than its surrounding normal kidney epithelial cells to recruit the CD4 + T cells. In vivo mouse model studies were consistent with the results by in vitro cell lines studies showing infiltrating T cells enhanced RCC cell proliferation. qRT-PCR and western blot exhibited that CD4 + T cells could enhance RCC cell proliferation via activating YBX1/HIF2α signaling pathway. Furthermore, CD4 + T cells functioned through inducing TGFβ1 expression. In a word, infiltrating CD4 + T cells promoted TGFβ1 expression in both RCC and T cells and regulated RCC cells proliferation via modulating TGFβ1/YBX1/ HIF2α signals.