Helical Dirac fermions - charge carriers that behave as massless relativistic particles with a quantum magnetic moment that is locked to its direction of motion - are proposed to be the key to realizing fundamentally new phenomena in condensed matter physics [1, 2]. Prominent examples include the anoma- lous quantization of magneto-electric coupling [3–6], half-fermion states that are their own anti-particle [7, 8], and charge fractionalization in a Bose-Einstein condensate [9], all of which are not possible with conventional Dirac fermions of the graphene variety [10]. Helical Dirac fermions are challenging to find, and so far remain undiscovered, because they are forbidden to exist in ordinary Dirac materials such as graphene [10] or bismuth [11], and because the nec- essary spin-sensitive measurements are lacking. It has recently been proposed that helical Dirac fermions may exist at the edges of certain types of topo- logically ordered insulators [3, 4, 12], and that their peculiar properties may be accessed provided the insulator is tuned into a topological transport regime [3–9]. However, helical Dirac fermions have not been observed in existing topo- logical insulators [13–18] to date because conventional electrical gating based tuning techniques that work for graphene [10, 19] cannot be applied and the necessary spin-resolved photoelectron detections do not exist. Here we report the first realization of a tunable topological insulator in a bismuth based class by combining spin- and momentum-resolved spectroscopies, bulk charge compen- sation and surface control. Our results reveal a spin-momentum locked Dirac cone that is nearly 100% spin-polarized, which exhibits a tunable topological fermion density in the vicinity of the Kramers’ point and can be driven to the long-sought topological transport regime. The observed topological nodal Dirac ground state is found to be protected even up to room temperature (300 K). Our results on Bi$_{2}$$_{−}$$_{δ}$Ca$_{δ}$Se$_{3}$.NO$_{2}$ pave the way for future transport based studies of topological insulators, and possible room temperature applications of protected spin-polarized edge channels in microelectronics technology. Unlike conventional Dirac fermions as in graphene, helical Dirac fermions possess a net spin and are guaranteed to be conducting because of time-reversal symmetry [3–5], allowing the unique possibility to carry spin currents without heat dissipation. However, the most important difference lies in the topological properties of helical Dirac fermion systems [3– 2