Over the past few decades, the classification of oncogenes or tumor suppressor genes has been an important topic in cancer biology. However, it is difficult to classify some genes. Heterogeneous nuclear ribonucleoprotein K (HNRNPK) is a nucleic acid-binding protein, which is involved in the regulation of gene expression, signal transduction and many other cellular processes. In recent years, it has been found that HNRNPK is overexpressed in many types of tumors, and its overexpression is negatively correlated with the prognosis of patients, suggesting that HNRNPK may play a role as an oncogene in tumorigenesis. In contrast, however, HNRNPK has also been considered as a tumor suppressor gene in acute myeloid leukemia (AML). Therefore, in this article we summarize and discuss the recent progress in the molecular functions and regulatory mechanisms of HNRNPK in tumorigenesis and progression.