Context. Transmission spectroscopy serves as a valuable tool for probing atmospheric absorption features in the terminator regions of exoplanets. Stellar surface heterogeneity can introduce wavelength-dependent contamination that complicates the interpretation of planetary spectra. Aims. We aim to investigate the atmosphere of the warm sub-Saturn NGTS-5b through optical transmission spectroscopy. Methods. Two transits were observed with the low-resolution Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) on the 10.4 m Gran Telescopio Canarias (GTC). Chromatic transit light curves were modeled to derive optical transmission spectra, and multiple Bayesian spectral retrievals were performed to characterize the atmospheric properties. Results. Model comparisons provide strong evidence for contamination from unocculted stellar spots. A joint retrieval of the transmission spectra, assuming equilibrium chemistry, indicates a relatively clear atmosphere with a subsolar C/O ratio of <0.22 (90% upper limit) and a low metallicity of 0.10−0.05+0.34× solar solar. Retrievals assuming free chemistry yield strong evidence for the presence of H2O, with its abundance constrained to log XH2O = −0.79−0.17+0.14. However, the abundances of other species remain unconstrained due to limited optical wavelength coverage. Conclusions. The discrepancies between the two NGTS-5b transit spectra can be attributed to varying levels of stellar contamination. NGTS-5b thus appears to host a relatively clear, water-rich atmosphere, pending confirmation from additional observations of molecular bands in the infrared.