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Bacteriophages exert strong selection on their bacterial hosts to evolve resistance. At the same time, the fitness costs on bacteria following phage resistance may change their virulence, which may affect the therapeutic outcomes of phage therapy. In this study, we set out to assess the costs of phage resistance on the <i>in vitro</i> virulence of priority 1 nosocomial pathogenic bacterium, <i>Acinetobacter baumannii</i>. By subjecting phage-resistant variant Ev5-WHG of <i>A. baumannii</i> WHG40004 to several <i>in vitro</i> virulence profiles, we found that its resistance to phage is associated with reduced fitness in host microenvironments. Also, the mutant exhibited impaired adhesion and invasion to mammalian cells, as well as increased susceptibility to macrophage phagocytosis. Furthermore, the whole-genome sequencing of the mutant revealed that there exist multiple mutations which may play a role in phage resistance and altered virulence. Altogether, this study demonstrates that resistance to phage can significantly alter phenotypes associated with virulence in <i>Acinetobacter baumannii</i>.