Cystic fibrosis (CF) airways disease represents an example of polymicrobial infection whereby different bacterial species can interact and influence each other. In CF patients Staphylococcus aureus is often the initial pathogen colonizing the lungs during childhood, while Pseudomonas aeruginosa is the predominant pathogen isolated in adolescents and adults. During chronic infection, P. aeruginosa undergoes adaptation to cope with antimicrobial therapy, host response and co-infecting pathogens. However, S. aureus and P. aeruginosa often co-exist in the same niche influencing the CF pathogenesis. The goal of this study was to investigate the reciprocal interaction of P. aeruginosa and S. aureus and understand the influence of P. aeruginosa adaptation to the CF lung in order to gain important insight on the interplay occurring between the two main pathogens of CF airways, which is still largely unknown. P. aeruginosa reference strains and eight lineages of clinical strains, including early and late clonal isolates from different patients with CF, were tested for growth inhibition of S. aureus. Next, P. aeruginosa/S. aureus competition was investigated in planktonic co-culture, biofilm, and mouse pneumonia model. P. aeruginosa reference and early strains, isolated at the onset of chronic infection, outcompeted S. aureus in vitro and in vivo models of co-infection. On the contrary, our results indicated a reduced capacity to outcompete S. aureus of P. aeruginosa patho-adaptive strains, isolated after several years of chronic infection and carrying several phenotypic changes temporally associated with CF lung adaptation. Our findings provide relevant information with respect to interspecies interaction and disease progression in CF.