  Ultraluminous X-ray sources (ULXs) are bright X-ray sources in nearby
galaxies not associated with the central supermassive black hole. Their
luminosities imply they are powered by either an extreme accretion rate onto a
compact stellar remnant, or an intermediate mass ($\sim100-10^5$ M$_{\odot}$)
black hole. The recent detection of coherent pulsations coming from three
bright ULXs demonstrates that some of these sources are powered by accretion
onto a neutron star, implying accretion rates significantly in excess of the
Eddington limit, a high degree of geometric beaming, or both. The physical
challenges associated with the high implied accretion rates can be mitigated if
the neutron star surface field is high - in the magnetar regime ($10^{14}$ G),
since this suppresses the electron scattering cross section, reducing the
radiation pressure that chokes off accretion for high luminosities. One of the
few ways to determine surface magnetic fields is through the detection of
cyclotron resonance scattering features (CRSFs) produced by the transition of
charged particles between quantized Landau levels. To date, CRSFs have only
been detected in Galactic accreting pulsars. Here we present the detection at
3.8-{\sigma} significance of a strong absorption line at a rest-frame energy of
4.5 keV in the Chandra spectrum of a ULX in M51. We find that this feature is
likely to be a CRSF produced by the strong magnetic field of a neutron star.
Assuming scattering off electrons, the magnetic field strength is implied to be
$\sim10^{11}$ G, however the line is narrower than any electron CRSFs
previously observed, and assuming thermal broadening, the implied temperature
is significantly cooler than the accretion disk or column. The line shape is,
however, consistent with a proton resonance scattering feature, implying that
the neutron star has a magnetic field near the surface of B$\sim10^{15}$ G.
