  We demonstrate single atom anisotropic magnetoresistance on the surface of a
topological insulator, arising from the interplay between the helical
spin-momentum-locked surface electronic structure and the hybridization of the
magnetic adatom states. Our first-principles quantum transport calculations
based on density functional theory for Mn on Bi$_2$Se$_3$ elucidate the
underlying mechanism. We complement our findings with a two dimensional model
valid for both single adatoms and magnetic clusters, which leads to a proposed
device setup for experimental realization. Our results provide an explanation
for the conflicting scattering experiments on magnetic adatoms on topological
insulator surfaces, and reveal the real space spin texture around the magnetic
impurity.
