  We investigate the behavior of superstring disk scattering amplitudes in the
presence of a soft external momentum at finite string tension. We prove that
there are no $\alpha'$-corrections to the field theory form of the subleading
soft factor $S^{(1)}$. At the end of this work, we also comment on the
possibility to find the corresponding subleading soft factors in closed string
theory using our result and the KLT relations.
