Wastewater treatment plants (WWTPs) represent an important reservoir of antibiotic resistance determinants. Although many studies have been conducted to evaluate resistance profiles in Enterobacteriaceae isolates from this setting, the dynamics of this phenomenon are poorly known to the bacterium Pseudomonas aeruginosa. Here we aimed to evaluate the resistance profiles and the production of AmpC β-lactamase in P. aeruginosa isolates from a domestic full-scale WWTP. Samples of the raw sewage and effluent were collected and the bacterium P. aeruginosa was isolated on cetrimide agar. Susceptibility to β-lactams, fluoroquinolones and aminoglycosides was evaluated by the disc diffusion method, and the presence of AmpC β-lactamase was investigated phenotypically and by molecular method. We recovered 27 isolates of P. aeruginosa. Of these, 81.5% were susceptible to all antimicrobials tested. However, a considerable rate of resistance to carbapenems (11%) was found among the isolates. Twenty-two isolates were positive in the phenotypic test for inducible AmpC β-lactamase but the blaampc gene was only identified in four isolates, suggesting the presence of other independent resistance mechanisms besides this β-lactamase. In summary, we have shown that P. aeruginosa isolates from a domestic WWTP represents a potential reservoir of blaampC genes and other resistance determinants, including those that result in low susceptibility to carbapenems and aminoglycosides.