Abstract The neutral atomic hydrogen (H i ) mass function (H i MF) describes the distribution of the H i content of galaxies at any epoch; its evolution provides an important probe of models of galaxy formation and evolution. Here, we report Giant Metrewave Radio Telescope H i 21 cm spectroscopy of blue star-forming galaxies at z ≈ 0.20–0.42 in the Extended Groth Strip, which has allowed us to determine the scaling relation between the average H i mass ( M H i ) and the absolute B -band magnitude ( M B ) of such galaxies at z ≈ 0.35, by stacking the H i 21 cm emission signals of galaxy subsamples in different M B ranges. We combine this M H i − M B scaling relation (with a scatter assumed to be equal to that in the local universe) with the known B -band luminosity function of star-forming galaxies at these redshifts to determine the H i MF at z ≈ 0.35. We show that the use of the correct scatter in the M H i − M B scaling relation is critical for an accurate estimate of the H i MF. We find that the H i MF has evolved significantly from z ≈ 0.35 to z ≈ 0, i.e., over the last 4 Gyr, especially at the high-mass end. High-mass galaxies, with M H i ≳ 10 10 M ⊙ , are a factor of ≈3.4 less prevalent at z ≈ 0.35 than at z ≈ 0. Conversely, there are more low-mass galaxies, with M H i ≈ 10 9 M ⊙ , at z ≈ 0.35 than in the local universe. While our results may be affected by cosmic variance, we find that massive star-forming galaxies have acquired a significant amount of H i through merger events or accretion from the circumgalactic medium over the past 4 Gyr.