Pneumococcal serine-rich repeat protein (PsrP) is a putative adhesin encoded in the Streptococcus pneumoniae pathogenicity island psrP-secY2A2. Challenge of mice with serotype 4, strain TIGR4, and the isogenic mutants T4ΔpsrP and T4ΔpsrP-secY2A2 determined that PsrP was required for bacterial persistence in the lungs but not for colonization in the nasopharynx or replication in the bloodstream during sepsis. In vitro experiments corroborated this anatomical site-specific role; psrP mutants failed to bind to A549 and LA-4 lung cells, yet adhered normally to human nasopharyngeal epithelial cells and to cells from human and rodent capillary endothelial cell lines. We determined that the amino terminus of PsrP mediated adhesion. Microspheres coated with recombinant PsrPSRR1-BR (rPsrP SRR1-BR) adhered to A549 cells, and moreover, preincubation of cells with rPsrPSRR1-BR inhibited TIGR4 adhesion in vitro. Antibodies against rPsrPSRR1-BR also neutralized PsrP function; antiserum against rPsrPSRR1-BR blocked TIGR4 adhesion in vitro and, following passive immunization, it protected mice against challenge. We conclude that PsrP is an adhesin required for bacterial persistence in the lungs and that rPsrPSRR1-BR is a protective antigen. © 2008 by the Infectious Diseases Society of America. All rights reserved.