  Classical Cepheid variable stars are crucial calibrators of the cosmic
distance scale thanks to a relation between their pulsation periods and
luminosities. Their archetype, {\delta} Cephei, is an important calibrator for
this relation. In this paper, we show that {\delta} Cephei is a spectroscopic
binary based on newly-obtained high-precision radial velocities. We combine
these new data with literature data to determine the orbit, which has period
2201 days, semi-amplitude 1.5 km/s, and high eccentricity (e = 0.647). We
re-analyze Hipparcos intermediate astrometric data to measure {\delta} Cephei's
parallax ($\varpi = 4.09 \pm 0.16$ mas) and find tentative evidence for an
orbital signature, although we cannot claim detection. We estimate that Gaia
will fully determine the astrometric orbit. Using the available information
from spectroscopy, velocimetry, astrometry, and Geneva stellar evolution models
($M_{\delta Cep} ~ 5.0 - 5.25 M_\odot$), we constrain the companion mass to
within $0.2 < M_2 < 1.2 M_\odot$. We discuss the potential of ongoing and
previous interactions between the companion and {\delta} Cephei near pericenter
passage, informing reported observations of circumstellar material and
bow-shock. The orbit may have undergone significant changes due to a
Kozai-Lidov mechanism driven by the outer (visual and astrometric) companion HD
213307. Our discovery of {\delta} Cephei's nature as a spectroscopic binary
exposes a hidden companion and reveals a rich and dynamical history of the
archetype of classical Cepheid variables.
