Nasal colonization by Staphylococcus aureus is the major risk factor for disease and transmission. Epidemiological studies have reported a reduced risk of S. aureus carriage in immunocompetent but not in immunocompromised children colonized by Streptococcus pneumoniae . We investigate the hypothesis that the immune response to pneumococcal colonization affects S. aureus colonization. We demonstrate that pneumococcal colonization in mice inhibits subsequent S. aureus acquisition in an antibody-dependent manner and elicits antibody that cross-reacts with S. aureus . We identify the staphylococcal target of cross-reactive antibody as 1-pyrroline-5-carboxylate dehydrogenase (P5CDH), and the homologous immunogen in S. pneumoniae as SP_1119, both of which are conserved dehydrogenases. These antigens are necessary and sufficient to inhibit the acquisition of S. aureus colonization in a mouse model. Our findings demonstrate that immune-mediated cross-reactivity between S. pneumoniae and S. aureus protects against S. aureus nasal acquisition and thus reveal a paradigm for identifying protective antigens against S. aureus .