In environmental settings, biofilms represent the common way of life of microorganisms. Salmonella enterica serovar Enteritidis, the most frequent cause of gastroenteritis in developed countries, produces a biofilm whose matrix is mainly composed of curli fimbriae and cellulose. In contrast to other bacterial biofilms, no proteinaceous compound has been reported to participate in the formation of this matrix. Here, we report the discovery of BapA, a large cell-surface protein required for biofilm formation by S. Enteritidis. Deletion of bapA caused the loss of the capacity to form a biofilm whereas the overexpression of a chromosomal copy of bapA increased the biofilm biomass formation. We provide evidence that overproduction of curli fimbriae and not cellulose can compensate for the biofilm deficiency of a bapA mutant strain. BapA is secreted through a type I protein secretion system (BapBCD) situated downstream of the bapA gene and was found to be loosely associated with the cell surface. Experiments with mixed bacterial populations positive or negative for BapA showed that BapA minus cells are not recruited into the biofilm matrix. The expression of bapA is coordinated with that of genes encoding curli fimbriae and cellulose, through the action of csgD. Studies on the contribution of BapA to S. Enteritidis pathogenesis revealed that orally inoculated animals with a bapA-deficient strain survived longer than those inoculated with the wild-type strain. Also, a bapA mutant strain showed a significantly lower colonization rate at the intestinal cell barrier and consequently a decreased efficiency for organ invasion compared with the wild-type strain. Taken together, these data demonstrate that BapA contributes both to biofilm formation and invasion through the regular Salmonella infection route.