We investigate Zeeman relaxation in cold Sb( 4 S ◦/2)–He collisions in a magnetic field. Ensembles of >10 13 laser-ablated Sb atoms are cooled in cryogenic 4 He buffer gas to 800 mK and inelastic collisions are observed to equilibrate the mJ -state distribution to the translational temperature. The ratio γ of momentum transfer to inelastic collision rates is measured to be 9.1 × 10 2 . We also perform quantum scattering calculations of Sb– 4 He collisions, based on ab initio interaction potentials, that demonstrate significant anisotropy of the ground state induced by the spin-orbit interaction. Agreement is obtained between theory and experiment with a ≈10% increase in the ab initio potential depth. This work suggests that buffer-gas-cooled pnictogen atoms lighter than