  We present $81$ near-infrared (NIR) spectra of $30$ Type II supernovae (SNe
II) from the Carnegie Supernova Project-II (CSP-II), the largest such dataset
published to date. We identify a number of NIR features and characterize their
evolution over time. The NIR spectroscopic properties of SNe II fall into two
distinct groups. This classification is first based on the strength of the He I
$\lambda1.083\,\mu$m absorption during the plateau phase; SNe II are either
significantly above (spectroscopically strong) or below $50$ angstroms
(spectroscopically weak) in pseudo equivalent width. However between the two
groups, other properties, such as the timing of CO formation and the presence
of Sr II, are also observed. Most surprisingly, the distinct weak and strong
NIR spectroscopic classes correspond to SNe II with slow and fast declining
light curves, respectively. These two photometric groups match the modern
nomenclature of SNe IIP and IIL. Including NIR spectra previously published, 18
out of 19 SNe II follow this slow declining-spectroscopically weak and fast
declining-spectroscopically strong correspondence. This is in apparent
contradiction to the recent findings in the optical that slow and fast
decliners show a continuous distribution of properties. The weak SNe II show a
high-velocity component of helium that may be caused by a thermal excitation
from a reverse-shock created by the outer ejecta interacting with the red
supergiant wind, but the origin of the observed dichotomy is not understood.
Further studies are crucial in determining whether the apparent differences in
the NIR are due to distinct physical processes or a gap in the current data
set.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics