Contrast medium (CM) is widely used in cardiac catheterization; however, it may induce acute kidney injury or renal failure, although the underlying mechanism remains to be elucidated. MicroRNA-21 (miR-21) is involved in renal disease and has been indicated to regulate cellular apoptosis and fibrosis, although its role in CM-induced renal cell injury is unknown. The present study examined the expression and potential targets of miR-21 in human renal proximal tubular epithelial (HK-2) cells following CM treatment. CM induced renal cell apoptosis and decreased miR-21 expression. The expression level of the apoptosis regulator protein, B-cell lymphoma 2 (Bcl-2) was upregulated, whereas that of the apoptosis regulator, Bcl-2-associated X protein (Bax) was downregulated upon transfection of miR-21 mimics; miR-21 overexpression additionally directly inhibited the expression of programmed cell death protein 4 (PDCD4), as determined by a dual luciferase reporter assay, and PDCD4 silencing reduced the rate of HK-2 cell apoptosis. The results of the present study indicated that miR-21 protected renal cells against CM-induced apoptosis by regulating PDCD4 expression.