Significance Ocean emissions of dimethyl sulfide (DMS) are a major precursor for the production and growth of aerosol particles, which can act as seeds for the formation of cloud droplets in the marine atmosphere with the subsequent impacts on Earth’s climate. Global aircraft observations indicate that DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a previously unrecognized molecule, which necessitates revisiting DMS oxidation chemistry in the marine atmosphere. We show through ambient observations and global modeling that a dominant loss pathway for HPMTF is uptake into cloud droplets. This loss process short circuits gas-phase oxidation and significantly alters the dynamics of aerosol production and growth in the marine atmosphere.