We have demonstrated the use of an optical frequency comb to coherently drive stimulated Raman transitions between terahertz-spaced atomic energy levels. Specifically, we have measured the 3d 2D3/2 - 3d 2D5/2 fine structure splitting of a single trapped 40Ca+ ion to be 1,819,599,021,534+/-8Hz, which is five times more accurate than previous measurements. In addition, Rabi oscillations with a contrast of 99.3(6)% and millisecond coherence time have been realized experimentally. Importantly, Raman transitions with level spacings ranging from sub-kHz to tens of THz range should be accessible, including hyperfine transitions in highly charged ions and spin-resolved rovibrational transitions in molecular ions.