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Intravenous gallium nitrate therapy is a novel therapeutic strategy deployed to combat chronic Pseudomonas aeruginosa biofilm infections in the lungs of cystic fibrosis (CF) patients by interfering with iron (Fe 3+ ) uptake. The therapy is a source of Ga 3+ , which competes with Fe 3+ for siderophore binding, subsequently disrupting iron metabolism and inhibiting biofilm proliferation in vivo . It was recently demonstrated that the Pseudomonas quinolone signal (PQS) can chelate Fe 3+ to assist in bacterial iron uptake. However, it is unknown whether exogenous gallium also targets [Fe(PQS) 3 ] uptake, which, in turn, would extend the mechanism of gallium therapy beyond siderophore competition, potentially supporting use of the therapy against P. aeruginosa mutants deficient in siderophore uptake proteins. To that end, the thermodynamic feasibility of iron-for-gallium cation exchange into [Fe(PQS) 3 ] was evaluated using quantum chemical density functional theory (DFT) modelling and verified experimentally using 1 H nuclear magnetic resonance (NMR). We demonstrate here that Ga 3+ can strongly bind to three PQS molecules and, furthermore, displace and substitute Fe 3+ from the native chelate pocket within PQS complexes, through a Trojan horse mechanism, retaining the key structural features present within the native ferric complex. As such, [Fe(PQS) 3 ] complexes, in addition to ferric-siderophore complexes, represent another target for gallium therapy.