jats:titleAbstract</jats:title> jats:pTOI-270 d is a temperate sub-Neptune discovered by the Transiting Exoplanet Survey Satellite (TESS) around a bright (jats:italicJ</jats:italic> = 9.1 mag) M3V host star. With an approximate radius of 2 jats:italicR</jats:italic> jats:sub⊕</jats:sub> and equilibrium temperature of 350 K, TOI-270 d is one of the most promising small exoplanets for atmospheric characterization using transit spectroscopy. Here we present a primary transit observation of TOI-270 d made with the Hubble Space Telescope Wide Field Camera 3 (WFC3) spectrograph across the 1.126–1.644 jats:italicμ</jats:italic>m wavelength range, and a 95% credible upper limit of 8.2 × 10jats:sup−14</jats:sup> erg sjats:sup−1</jats:sup> cmjats:sup−2</jats:sup> Åjats:sup−1</jats:sup> arcsecjats:sup−2</jats:sup> for the stellar Lyjats:italicα</jats:italic> emission obtained using the Space Telescope Imaging Spectrograph. The transmission spectrum derived from the TESS and WFC3 data provides evidence for molecular absorption by a hydrogen-rich atmosphere at 4jats:italicσ</jats:italic> significance relative to a featureless spectrum. The strongest evidence for any individual absorber is obtained for Hjats:sub2</jats:sub>O, which is favored at 3jats:italicσ</jats:italic> significance. When retrieving on the WFC3 data alone and allowing for the possibility of a heterogeneous stellar brightness profile, the detection significance of Hjats:sub2</jats:sub>O is reduced to 2.8jats:italicσ</jats:italic>. Further observations are therefore required to robustly determine the atmospheric composition of TOI-270 d and assess the impact of stellar heterogeneity. If confirmed, our findings would make TOI-270 d one of the smallest and coolest exoplanets to date with detected atmospheric spectral features.</jats:p>