  We report on observations of the pulsar / Be star binary system PSR
J2032+4127 / MT91 213 in the energy range between 100 GeV and 20 TeV with the
VERITAS and MAGIC imaging atmospheric Cherenkov telescope arrays. The binary
orbit has a period of approximately 50 years, with the most recent periastron
occurring on 2017 November 13. Our observations span from 18 months prior to
periastron to one month after. A new, point-like, gamma-ray source is detected,
coincident with the location of PSR J2032+4127 / MT91 213. The gamma-ray light
curve and spectrum are well-characterized over the periastron passage. The flux
is variable over at least an order of magnitude, peaking at periastron, thus
providing a firm association of the TeV source with the pulsar / Be star
system. Observations prior to periastron show a cutoff in the spectrum at an
energy around 0.5 TeV. This result adds a new member to the small population of
known TeV binaries, and it identifies only the second source of this class in
which the nature and properties of the compact object are firmly established.
  We compare the gamma-ray results with the light curve measured with the X-ray
Telescope (XRT) on board the Neil Gehrels \textit{Swift} Observatory and with
the predictions of recent theoretical models of the system. We conclude that
significant revision of the models is required to explain the details of the
emission we have observed, and we discuss the relationship between the binary
system and the overlapping steady extended source, TeV J2032+4130.
