  We report on a NuSTAR observation of the recently discovered bright black
hole candidate MAXI J1535-571. NuSTAR observed the source on MJD 58003 (five
days after the outburst was reported). The spectrum is characteristic of a
black hole binary in the hard state. We observe clear disk reflection features,
including a broad Fe K$\alpha$ line and a Compton hump peaking around 30 keV.
Detailed spectral modeling reveals narrow Fe K$\alpha$ line complex centered
around 6.5 keV on top of the strong relativistically broadened Fe K$\alpha$
line. The narrow component is consistent with distant reflection from
moderately ionized material. The spectral continuum is well described by a
combination of cool thermal disk photons and a Comptonized plasma with the
electron temperature $kT_{\rm e}=19.7\pm{0.4}$ keV. An adequate fit can be
achieved for the disk reflection features with a self-consistent relativistic
reflection model that assumes a lamp-post geometry for the coronal illuminating
source. The spectral fitting measures a black hole spin $a>0.84$, inner disk
radius $R_{\rm in}<2.01~r_{\rm ISCO}$, and a lamp-post height
$h=7.2^{+0.8}_{-2.0} r_{\rm g}$ (statistical errors, 90% confidence),
indicating no significant disk truncation and a compact corona. Although the
distance and mass of this source are not currently known, this suggests the
source was likely in the brighter phases of the hard state during this NuSTAR
observation.
