Helical Dirac fermions are charge carriers that behave as massless relativistic particles with an intrinsic angular momentum (spin) locked to their translational momentum, a property desirable for spintronic and computing technologies. It has recently been proposed that such fermions may exist at the edges of certain types of topologically ordered insulators. Here, the realization and characterization of such a system is reported; the results reveal nearly 100 per cent spin polarization, even up to room temperature.