  We present the discovery and validation of a temperate sub-Neptune around the
nearby mid-M dwarf TIC 470381900 (TOI-1696), with a radius of $3.09 \pm 0.11
\,R_\oplus$ and an orbital period of $2.5 \,\rm{days}$, using a combination of
TESS and follow-up observations using ground-based telescopes. Joint analysis
of multi-band photometry from TESS, MuSCAT, MuSCAT3, Sinistro, and KeplerCam
confirmed the transit signal to be achromatic as well as refined the orbital
ephemeris. High-resolution imaging with Gemini/'Alopeke and high-resolution
spectroscopy with the Subaru/IRD confirmed that there are no stellar companions
or background sources to the star. The spectroscopic observations with IRD and
IRTF/SpeX were used to determine the stellar parameters, and found the host
star is an M4 dwarf with an effective temperature of $T_{eff} = 3185 \pm
76\,\rm{K}$ and a metallicity of [Fe/H] $=0.336 \pm 0.060 \,\rm{dex}$. The
radial velocities measured from IRD set a $2$-$\sigma$ upper limit on the
planetary mass to be $48.8 \,M_\oplus$. The large radius ratio ($R_p/R_\star
\sim 0.1$) and the relatively bright NIR magnitude ($J=12.2 \,\rm{mag}$) make
this planet an attractive target for further followup observations. TOI-1696b
is one of the planets belonging to the Neptunian desert with the highest
transmission spectroscopy metric discovered to date, making it an interesting
candidate for atmospheric characterizations with JWST.
