  We present a spectral analysis of three simultaneous NuSTAR and Swift/XRT
observations of the transient Be-neutron star binary KS 1947+300 taken during
its outburst in 2013/2014. These broad-band observations were supported by
Swift/XRT monitoring snap-shots every 3 days, which we use to study the
evolution of the spectrum over the outburst. We find strong changes of the
power-law photon index, which shows a weak trend of softening with increasing
X-ray flux. The neutron star shows very strong pulsations with a period of P ~
18.8 s. The 0.8-79 keV broad-band spectrum can be described by a power-law with
an exponential cutoff and a black-body component at low energies. During the
second observation we detect a cyclotron resonant scattering feature at 12.5
keV, which is absent in the phase-averaged spectra of observations 1 and 3.
Pulse phase-resolved spectroscopy reveals that the strength of the feature
changes strongly with pulse phase and is most prominent during the broad
minimum of the pulse profile. At the same phases the line also becomes visible
in the first and third observation at the same energy. This discovery implies
that KS 1947+300 has a magnetic field strength of B ~ 1.1e12 (1+z)G, which is
at the lower end of known cyclotron line sources.
