Antimicrobial resistance, a major consequence of diagnostic uncertainty and antimicrobial overprescription, is an increasingly recognized cause of severe infections, complications, and mortality worldwide with a huge impact on our society and on the health system. In particular, patients with compromised immune systems or pre-existing and chronic pathologies, such as cystic fibrosis (CF), are subjected to frequent antibiotic treatments to control the infections with the appearance and diffusion of multidrug resistant isolates. Therefore, there is an urgent need to address alternative therapies to counteract bacterial infections. Use of bacteriophages, the natural enemies of bacteria, can be a possible solution. The protocol detailed in this work describes the application of phage therapy against Pseudomonas aeruginosa infection in CF zebrafish embryos. Zebrafish embryos were infected with P. aeruginosa to demonstrate that phage therapy is effective against P. aeruginosa infections as it reduces lethality, bacterial burden and pro-inflammatory immune response in CF embryos.