Spin-momentum locking, which constrains spin orientation perpendicular to electron momentum, is attracting considerable interest for exploring various spin functionalities in semiconductors and topological materials. Efficient spin generation and spin detection have been demonstrated using the induced helical spin texture. Nevertheless, spin manipulation by spin-momentum locking remains a missing piece because, once bias voltage is applied to induce the current flow, the spin orientation must be locked by the electron momentum direction, thereby rendering spin phase control difficult. Herein, we demonstrate the spin-momentum locking-induced spin manipulation for ballistic electrons in a strong Rashba two-dimensional system. Electron spin rotates in a circular orbital motion for ballistically moving electrons, although spin orientation is locked towards the spin-orbit field because of the helical spin texture. This fact demonstrates spin manipulation by control of the electron orbital motion and reveals potential effects of the orbital degree of freedom on the spin phase for future spintronic and topological devices and for the processing of quantum information.