We present the first measurement of H I mass of star-forming galaxies in different large scale structure environments from a blind survey at z ~ 0.37. In particular, we carry out a spectral line stacking analysis considering 2875 spectra of colour-selected star-forming galaxies undetected in H I at 0.23 < z < 0.49 in the COSMOS field, extracted from the MIGHTEE-H I Early Science data cubes, acquired with the MeerKAT radio telescope. We stack galaxies belonging to different subsamples depending on three different definitions of large-scale structure environment: local galaxy overdensity, position inside the host dark matter halo (central, satellite, or isolated), and cosmic web type (field, filament, or knot). We first stack the full star-forming galaxy sample and find a robust H I detection yielding an average galaxy H I mass of $M_{\rm H \, {\small I}}=(8.12\pm 0.75)\times 10^9\, {\rm M}_\odot$ at ~11.8σ. Next, we investigate the different subsamples finding a negligible difference in MH I as a function of the galaxy overdensity. We report an H I excess compared to the full sample in satellite galaxies (MH I = (11.31 ± 1.22) × 109, at ~10.2σ) and in filaments (MH I = (11.62 ± 0.90) × 109. Conversely, we report non-detections for the central and knot galaxies subsamples, which appear to be H I-deficient. We find the same qualitative results also when stacking in units of H I fraction (fH I). We conclude that the H I amount in star-forming galaxies at the studied redshifts correlates with the large-scale structure environment.