The emergence of multidrug-resistant (MDR) strains is a major health problem worldwide. There is an urgent need for novelstrategies to combat bacterial infections caused by MDR strains likePseudomonas aeruginosaandAcinetobacter baumannii.Quorum sensing (QS) is a critical communication system in bacterial community controlling survival and virulence. Theawareness of the importance of QS in bacterial infections has stimulated research to identify QS inhibitors (QSIs) to defeatmicrobes. In this study, four FDA-approved drugs (besides azithromycin as positive QSI) were tested for potential QS inhibitionagainst clinicalA. baumanniiisolates andP. aeruginosa(PAO1) standard strain. The inhibitory effect of these drugs on virulencefactors of both microbes has been investigated. The studied virulence factors include biofilm formation, twitching and swarmingmotilities, proteolytic enzyme production, and resistance to oxidative stress. The four tested drugs (erythromycin, levamisole,chloroquine, and propranolol) inhibited QS inChromobacterium violaceumby 84, 72, 55.1, and 37.3%, respectively. They alsosignificantly inhibited virulence factors in both PAO1 andA. baumanniiat sub-inhibitory concentrations. These findings wereconfirmed by qRT-PCR and mice mortality test, where tested drugs highly repressed the expression ofabaIgene and showedsignificantly improved mice survival rates. In addition, molecular docking studies against AbaI and AbaR proteins of QS systeminA. baumanniirevealed the potential inhibition of QS by tested drugs. Beside their known activities, the tested drugs could be given new life as QSIs to combatA. baumanniinosocomial infections (alone or in combination with antimicrobials)