Ceftolozane-tazobactam is a new β-lactam/β-lactamase inhibitor combination approved by the U.S. Food and Drug Administration in 2019 for the treatment of hospital-acquired and ventilator-associated pneumonia. The combination is a particularly potent inhibitor of penicillin-binding proteins with higher affinity than other β-lactam agents. Persons with cystic fibrosis (pwCF) often harbour resistant Gram-negative bacteria in the airways and need antibiotics to prevent declining lung function. To test whether the introduction of ceftolozane-tazobactam in the period 2015–2020 led to a bacterial population level increase in cephalosporin resistance in a Danish CF population. In vitro, activity of ceftolozane-tazobactam was evaluated by susceptibility testing of clinical Pseudomonas aeruginosa isolated from pwCF from January 1, 2015, to June 1, 2020. Six thousand three hundred thirty two isolates collected from 210 adult pwCF were included. Thirty pwCF were treated with ceftolozane-tazobactam at least once. Ceftolozane-tazobactam exposure did not increase cephalosporin resistance on an individual or population level. However, resistance to ceftolozane-tazobactam was recorded despite no prior exposure in four pwCF. Compared to ceftazidime, ceftolozane-tazobactam had a better in vitro activity on P. aeruginosa. The percentage of non-mucoid P. aeruginosa isolates susceptible to ceftolozane-tazobactam were higher or equal to 5 other β-lactams. Ceftolozane-tazobactam expands the armamentaria against P. aeruginosa with acceptable levels for a selection of drug resistance.