Colistin is a last-resort antimicrobial agent for treating carbapenem-resistantAcinetobacter baumanniiinfections. The activation of PmrAB by several environmental signals induces colistin resistance in Gram-negative bacteria. This study investigated the molecular mechanisms of colistin resistance inA. baumanniiunder acidic conditions using wild-type (WT)A. baumannii17978, ΔpmrAand ΔpmrBmutants, andpmrA-complemented strains. ThepmrAorpmrBdeletion did not affect the growth ofA. baumanniiunder acidic or aerobic conditions.A. baumanniiunder acidic (pH 5.5) and high-iron (1 mM) conditions showed 32- and 8-fold increases in the minimum inhibitory concentrations (MICs) of colistin, respectively. The ΔpmrAand ΔpmrBmutants at pH 5.5 showed a significant decrease in colistin MICs compared to the WT strain at pH 5.5. No difference in colistin MICs was observed between WT and mutant strains under high-iron conditions. ThepmrCABexpression significantly increased in the WT strain at pH 5.5 compared to the WT strain at pH 7.0. ThepmrCexpression significantly decreased in two mutant strains at pH 5.5 compared to the WT strain at pH 5.5. The PmrA protein was expressed in the ΔpmrAstrain carrying ppmrA_FLAG plasmids at pH 5.5 but not at pH 7.0. Lipid A modification by the addition of phosphoethanolamine was observed in the WT strain at pH 5.5. In conclusion, this study demonstrated thatA. baumanniiunder acidic conditions induces colistin resistance via the activation ofpmrCABoperon and subsequent lipid A modification.