Clear cell renal cell carcinoma (ccRCC) metastasis may result from epithelial-to-mesenchymal transition and mesenchymal stem cells that contribute to the development of the primary tumor. In this study, it was demonstrated that microRNA-21 (miR-21) acts as an oncogenic driver of ccRCC. ccRCC spheres were isolated and it was shown that they exhibited cancer stem cell‑like properties, including the formation of self‑renewing spheres. Spheres showed increased expression of stem cell‑related transcription factors and epithelial‑mesenchymal transition (EMT) markers. In addition, secondary sphere formation capacity was assessed after miR‑21 transfection. miR‑21 accelerated the formation of ccRCC spheres, which shared molecular characteristics with the spontaneous ccRCC spheres. It was demonstrated that miR‑21 overexpression facilitates ccRCC sphere formation. Thus, a single miRNA may have an impact on the formation of highly tumorigenic cancer spheres in kidney cancer.