A prominent feature of topological insulators (TI) is the surface states comprising of spin-nondegenerate massless Dirac fermions. Recent technical advances have made it possible to address the surface transport properties of TI thin films by tuning the Fermi levels of both top

and bottom surfaces. In my talk, I am going to talk about our recent discovery of a novel planar Hall effect (PHE) from the TI surface, which results from an unknown resistivity anisotropy induced by an in-plane magnetic field. This effect is observed in dual-gated devices of bulk-insulating Bi_{2−x}Sb_{x}Te_{3} thin films, where the field-induced anisotropy presents a strong dependence on the gate voltage with a characteristic two-peak structure near the Dirac point. The origin of PHE is the peculiar time-reversal-breaking effect of an in-plane magnetic field, which anisotropically lifts the protection of surface Dirac fermions from backscattering. The observed PHE provides a useful tool to analyse and manipulate the topological protection of the TI surface.