The low‐molecular‐weight (LMW) reduced sulfur substances (RSS) composition of dissolved organic matter (DOM) was examined along the GEOTRACES US‐GP15 section in the Pacific Ocean. We demonstrate that LMW RSS constitutes a significant fraction of nonvolatile dissolved organic sulfur (DOS). While thiols such as glutathione were below our detection limit (300 pM), RSS containing two carbon (C) sulfur (S) bonds were present at concentrations in the hundreds of nM range. RSS accumulation was observed in subtropical waters. The most likely source of these RSS is microbial alteration of sulfurized DOM with production of secondary thioamidated metabolites. RSS are initially produced by cyanobacteria to mitigate copper and oxidative stress induced by UV‐B irradiance. A preferential remineralization of RSS over dissolved organic carbon (DOC) in the upper 350 m suggests a partial lability of LMW DOS. Deeper, homogeneous concentrations and C : S ratio indicate increasing stability of this LMW DOS.