The weak contacts between disulfide linkages and carbonyl groups are anticipated to be important in determining the structure and function of enzymes and proteins. However, the characteristics of the disulfide-carbonyl n → π* (nSS → π* C═O) interactions remain unexplored. Herein, we investigated the nSS → π* C═O interactions in the gas phase and in proteins. Rotational spectroscopic investigation of a model complex of allyl methyl disulfide with formaldehyde identified two structures, both of which are stabilized through a dominant nSS → π* C═O interaction. Surveys of the Protein Data Bank revealed the occurrence of 18 675 nSS → π* C═O interactions associated with 15 320 disulfide bonds in 7105 protein structures. Further theoretical analyses characterize the bonding nature of the nSS → π* C═O interactions. This study provides an in-depth understanding of the stabilizing effect of the nSS → π* C═O interactions in small molecular complexes and biomacromolecules.