Spin polarization of a topological surface state for GeBi2Te4, the newly discovered three-dimensional topological insulator, has been studied by means of state-of-the-art spin- and angle-resolved photoemission spectroscopy. It has been revealed that the disorder in the crystal has a minor effect on the surface-state spin polarization, which is 70% near the Dirac point in the bulk energy gap region (∼180 meV). This finding promises not only to realize a highly spin-polarized surface-isolated transport but also to add functionality to its thermoelectric and thermomagnetic properties.