  The NASA space telescope $TESS$ is currently in the extended mission of its
all-sky search for new transiting planets. Of the thousands of candidates that
TESS is expected to deliver, transiting planets orbiting nearby M dwarfs are
particularly interesting targets since they provide a great opportunity to
characterize their atmospheres by transmission spectroscopy. We aim to validate
and characterize the new sub-Neptune-sized planet candidate TOI-2136.01
orbiting a nearby M dwarf ($d = 33.36 \pm 0.02$ pc, $T_{eff} = 3373 \pm 108$ K)
with an orbital period of 7.852 days. We use TESS data, ground-based multicolor
photometry, and radial velocity measurements with the InfraRed Doppler (IRD)
instrument on the Subaru Telescope to validate the planetary nature of
TOI-2136.01 and estimate the stellar and planetary parameters. We also conduct
high-resolution transmission spectroscopy to search for helium in its
atmosphere. We confirmed that TOI-2136.01 (now named as TOI-2136b) is a bona
fide planet with a planetary radius of $R_p = 2.2 \pm 0.07$ $R_{Earth}$ and a
mass of $M_p = 4.7^{+3.1}_{-2.6}$ $M_{Earth}$. We also search for helium 10830
\r{A} absorption lines and place an upper limit on the equivalent width of $<$
7.8 m\r{A} and on the absorption signal of $<$ 1.44 % with 95 % confidence.
TOI-2136b is a sub-Neptune transiting a nearby and bright star (J=10.8) and is
a potentially hycean planet, which is a new class of habitable planets with
large oceans under a H$_2$-rich atmosphere, making it an excellent target for
atmospheric studies to understand the formation, evolution, and habitability of
the small planets.
