Acinetobacter baumannii has been named by the World Health Organisation as the top priority pathogen requiring new therapeutic strategies. One of its key pathogenicity factors, the polysaccharide capsule, facilitates resistance by hampering antibiotic entry. We previously isolated and characterised bacteriophage øFG02, which targets AB900, a clinical A. baumannii isolate. Furthermore, we demonstrated that øFG02 uses the bacterial capsule as its receptor and described an AB900 mutant that achieved phage-resistance by disrupting its capsule production. After losing its capsule, the phage-resistant strain became resensitised to multiple antibiotics. Here, we aimed to leverage these preliminary observations into their preclinical translation. We used a murine model of bacteraemia to determine that once-a-day and twice-a-day administration of øFG02 are comparable in reduction of bacterial burdens. In addition, we observed the emergence of phage-resistant bacteria in every phage-treated mouse (n = 8), in a proportion of 11.9%. We randomly selected one phage-resistant isolate from each animal, sequenced their genomes, and found loss-of-function mutations in genes responsible for capsule production in all of them. Remarkably, all of the phage-resistant mutants had become resensitised to ceftazidime, pointing at the repeatability of the mechanism and trade-off of phage-resistance. We then confirmed the existence of synergy between phage øFG02 and ceftazidime in an in vitro platform. When working together, even subinhibitory doses of ceftazidime and øFG02 achieved bacterial clearance. In vivo, combinational treatment resulted in a ~4 log reduction in bacterial burden compared to antibiotic monotherapy. Our findings will help guide further preclinical trials of this combinational therapy.