With the growing threat of antimicrobial resistance (AMR), alternatives to conventional antibiotics are urgently needed. Here, we show that mRNA-based therapeutics encoding single-chain variable fragment (scFv) antibodies-small, targeted antibody derivatives-provide robust protection against Pseudomonas aeruginosa, a major multidrug-resistant pathogen. We target the bacterial type III secretion system (T3SS), a needle-like apparatus used by the pathogen to inject toxins into host cells. When delivered intravenously via lipid nanoparticles, the scFv-encoding mRNA prompts sustained protein production, overcoming the typically short half-life of small antibody fragments. This treatment mitigates lung inflammation, reduces bacterial load, and improves survival in clinically relevant models, including immunocompromised mice infected with multidrug-resistant, exoU-positive (highly cytotoxic) clinical isolates. We find that Fc-free scFv antibodies, consisting only of the antigen-binding domain, migrate more efficiently from the bloodstream to the lung epithelium-the primary site of infection-than their larger counterpart conjugated to an Fc domain. This enhanced tissue penetration results in superior therapeutic outcomes. Overall, mRNA-encoded, Fc-free antibody fragments represent a promising and versatile platform for combating life-threatening bacterial infections without relying on traditional antibiotics.