We have investigated the spin polarization and orbital configuration of the spin-split surface states in a nonsymmorphic semimetal HfSiS by spin-and angle-resolved photoelectron spectroscopy using polarized tunable photons combined with detailed theoretical calculations. We have found that the surface states surrounding thē X point with C 2v symmetry exhibit an exotic unidirectional spin texture coupled with d yz orbital. The observed unidirectional spin texture is well reproduced by the theoretical calculations. Our work clarifies the significance of orbital symmetry in the manipulation of spin orientations, providing an approach to realize the persistent spin texture for the prolonged spin-coherence time in spintronic applications.