It is standard practice to consider thorium as a traditional tetravalent d transition metal, belonging to the same group of elements as Ti, Zr, and Hf. We demonstrate, however, that the observed fcc crystal structure for thorium is in complete disagreement with this picture. Instead we show that it is the itinerant 5f electrons which stabilize the fcc phase. Thereby we have established a close connection between thorium and the heavier actinide metals (Pa-Pu), which has previously been overlooked.