Dextrin-colistin conjugates have been developed with the aim of achieving reduced clinical toxicity associated with colistin, also known as polymyxin E, and improved targeting to sites of bacterial infection. This study investigated the in vitro ability of such dextrin-colistin conjugates to bind and modulate bacterial lipopolysaccharide (LPS), and how this binding affects its biological activity. These results showed that colistin and amylase-activated dextrin-colistin conjugate to a lesser extent induced aggregation of LPS to form a stacked bilayer structure with characteristic dimensions, although this did not cause any substantial change in its secondary structure. In biological studies, both colistin and dextrin-colistin conjugate effectively inhibited LPS-induced hemolysis and tumor necrosis factor α (TNFα) secretion in a concentration-dependent manner, but only dextrin-colistin conjugate showed no additive toxicity at higher concentrations. This study provides the first direct structural experimental evidence for the binding of dextrin-colistin conjugates and LPS and gives insight into the mode of action of dextrin-colistin conjugates.