Three-dimensional topological insulators(1-3) represent a new quantum phase of matter with spin-polarized surface states(4,5) that are protected from backscattering. The static electronic properties of these surface states have been comprehensively imaged by both photoemission(4-8) and tunnelling(9,10) spectroscopies. Theorists have proposed that topological surface states can also exhibit novel electronic responses to light, such as topological quantum phase transitions(11-13) and spin-polarized electrical currents(14,15). However, the effects of optically driving a topological insulator out of equilibrium have remained largely unexplored experimentally, and no photocurrents have been measured. Here, we show that illuminating the topological insulator Bi2Se3 with circularly polarized light generates a photocurrent that originates from topological helical Dirac fermions, and that reversing the helicity of the light reverses the direction of the photocurrent. We also observe a photocurrent that is controlled by the linear polarization of light and argue that it may also have a topological surface state origin. This approach may allow the probing of dynamic properties of topological insulators(11-15) and lead to novel opto-spintronic devices(16).