The study investigates the atmospheric composition of the sub-Neptune exoplanet TOI-270 d, a candidate Hycean world, using JWST transmission spectroscopy. The observations were conducted with the JWST NIRSpec G395H instrument in the 2.7-5.2 μm range, complemented by previous HST WFC3 observations in the 1.1-1.6 μm range. The analysis reveals strong signatures of CH4 and CO2 at 3.8-4.9σ and 2.9-3.9σ confidence, respectively, indicating a H2-rich atmosphere with abundant CH4 and CO2 at mixing ratios of ~0.1-1%. No NH3 was detected, consistent with predictions for a Hycean world. Evidence for CS2 was also found at 2.3-3.0σ confidence, and potential hints for H2O at 1.6-4.4σ confidence, depending on the data analysis approach. The spectrum does not provide strong constraints on cloud or haze presence, nor does it reveal effects of stellar heterogeneities. For the inner planet TOI-270 b, the spectrum shows some evidence of an H2-rich atmosphere but requires more observations to confirm these results. The findings support the classification of TOI-270 d as a Hycean world, with a potentially habitable ocean under a H2-rich atmosphere, consistent with previous predictions and observations of K2-18 b.The study investigates the atmospheric composition of the sub-Neptune exoplanet TOI-270 d, a candidate Hycean world, using JWST transmission spectroscopy. The observations were conducted with the JWST NIRSpec G395H instrument in the 2.7-5.2 μm range, complemented by previous HST WFC3 observations in the 1.1-1.6 μm range. The analysis reveals strong signatures of CH4 and CO2 at 3.8-4.9σ and 2.9-3.9σ confidence, respectively, indicating a H2-rich atmosphere with abundant CH4 and CO2 at mixing ratios of ~0.1-1%. No NH3 was detected, consistent with predictions for a Hycean world. Evidence for CS2 was also found at 2.3-3.0σ confidence, and potential hints for H2O at 1.6-4.4σ confidence, depending on the data analysis approach. The spectrum does not provide strong constraints on cloud or haze presence, nor does it reveal effects of stellar heterogeneities. For the inner planet TOI-270 b, the spectrum shows some evidence of an H2-rich atmosphere but requires more observations to confirm these results. The findings support the classification of TOI-270 d as a Hycean world, with a potentially habitable ocean under a H2-rich atmosphere, consistent with previous predictions and observations of K2-18 b.