A predominantly T-helper type 2 (Th2) immune response is critical in the prognosis of pulmonary Pseudomonas aeruginosa infection. But the mucosal and systemic immune responses can be influenced by the intestinal microbiota. We assessed the effect of microbiota compositional changes induced by a diet enriched in 5% acidic oligosaccharides derived from pectin (pAOS) on the immune response and outcome of chronic pulmonary P. aeruginosa infection in mice. pAOS promoted Th1 polarization by increasing interferon γ release, upregulating t-bet gene expression, decreasing interleukin 4 secretion, and downregulating gata3 gene expression. pAOS also sustained the release of keratinocyte chemoattractant, recruited polynuclear leukocytes and macrophages, stimulated M1 macrophage activation and interleukin 10 release, and decreased tumor necrosis factor α release in the lung. These effects led to increased bacterial clearance after the first and second P. aeruginosa infections. pAOS modified the intestinal microbiota by stimulating the growth of species involved in immunity development, such as Bifidobacterium species, Sutturella wadsworthia, and Clostridium cluster XIVa organisms, and at the same time increased the production of butyrate and propionate. These results suggest that pAOS may have beneficial effects by limiting the number and severity of pulmonary exacerbations in patients chronically infected with P. aeruginosa, such as individuals with cystic fibrosis.