Traditional Chinese Medicine (TCM) has been increasingly studied for antitumor activities. Icaritin, a hydrolytic product of icariin, is an effective ingredient of the traditional Chinese herb epimedium with multiple pharmacological activities. Among them, the antitumor activity of icaritin has been widely studied and reported in tumors both in vitro and in vivo. While its exact antitumor mechanisms await revelation, icaritin has been found to regulate several key molecules and pathways concerning cell fate, including CDK-dependent pathways, mitogen-activated protein kinases (MAPKs), the serine-threonine kinase AKT, signal transducer and activator of transcription 3 (STAT3), and p53. The ability to induce cellular oxidative stress also contributes to its antitumor activity. This review outlines the results of key investigations focusing on the antitumor effects and mechanisms of icaritin.