  We report the discovery of a quiescent galaxy at $z=2.34$ with a stellar mass
of only $M_\star = 9.5^{+1.8}_{-1.2} \times 10^{8} \mathrm{M}_\odot$, based on
deep JWST/NIRSpec spectroscopy. This is the least massive quiescent galaxy
found so far at high redshift. We use a Bayesian approach to model the spectrum
and photometry, and find the target to have been quiescent for 0.6 Gyr with a
mass-weighted average stellar age of 0.8-1.7 Gyr (dominated by systematics).
The galaxy displays an inverse colour gradient with radius, consistent with
environment-driven quenching. Based on a combination of spectroscopic and
robust (medium- and broad-band) photometric redshifts, we identify a galaxy
overdensity near the location of the target (5-$\sigma$ above the background
level at this redshift). We stress that had we been specifically targetting
galaxies within overdensities, the main target would not have been selected on
photometry alone; therefore, environment studies based on photometric redshifts
are biased against low-mass quiescent galaxies. The overdensity contains three
spectroscopically confirmed, massive, old galaxies ($M_\star = 8-17 \times
10^{10} \mathrm{M}_\odot$). The presence of these evolved systems points to
accelerated galaxy evolution in overdensities at redshifts z > 2, in agreement
with previous works. In projection, our target lies only 35 pkpc away from the
most massive galaxy in this overdensity (spectroscopic redshift z = 2.349)
which is located close to overdensity's centre. This suggests the low-mass
galaxy was quenched by environment, making it possibly the earliest evidence
for environment-driven quenching to date.
