For a long time cyclin dependent kinase 5 (Cdk5) was thought to be of exclusive importance in neuronal cells. However, recently increasing evidence suggests a function of Cdk5 in cancer progression. In the present study, we examined the role of Cdk5 in hepatocellular carcinoma (HCC), a highly chemoresistant cancer with poor prognosis. Consequently, development of novel targeted therapies for HCC is of paramount clinical importance. Analysis of human HCC patient samples showed an increased expression of Cdk5 as compared to normal liver tissues. Functional ablation of Cdk5 significantly decreases HCC cell proliferation and clonogenic survival, and reduces cell motility and invasion. Of note, genetic as well as pharmacologic inhibition of Cdk5 also shows in vivo efficacy in a HCC xenograft mouse model. Investigating the mechanism behind these functional effects revealed that Cdk5 is most active in the nucleus of cells in G2/M phase. In this cell cycle phase DNA damage response takes place, which is affected by Cdk5 inhibition. Furthermore, Cdk5 leads to phosphorylation of Ataxia Telangiectasia Mutated (ATM) and thereby influence its downstream signaling. Importantly, combination of Cdk5 inhibition with different DNA damage inducing chemotherapeutics or the first-line systemic drug sorafenib inhibits synergistically HCC tumor progression. In conclusion, we introduce: 1. Cdk5 as a novel drugable target for treatment of HCC 2. The combination of Cdk5 inhibition and DNA damage agents as a novel therapeutic approach 3. An increased efficacy of sorafenib treatment by combing with Cdk5 inhibition