  We present VLT/SPHERE adaptive optics imaging in Y$-$, J$-$, H$-$, and
K-bands of the HD 100453 system and the discovery of a two-armed spiral
structure in a disk extending to 0.37" ($\sim$42 AU) from the star, with highly
symmetric arms to the Northeast and Southwest. Inside of the spiral arms, we
resolve a ring of emission from 0.18"-0.25" ($\sim$21-29 AU). By assuming that
the ring is intrinsically circular we estimate an inclination of $\sim$34$^{o}$
from face-on. We detect dark crescents on opposite sides (NW and SE) which
begin at 0.18" and continue to radii smaller than our inner working angle of
0.15", which we interpret as the signature of a gap at $\lesssim$21 AU that has
likely been cleared by forming planets. We also detect the $\sim$120 AU
companion HD 100453 B, and by comparing our data to 2003 HST/ACS and VLT/NACO
images we estimate an orbital period of $\sim$850 yr. We discuss what
implications the discovery of the spiral arms and finer structures of the disk
may have on our understanding of the possible planetary system in HD 100453,
and how the morphology of this disk compares to other related objects.
