Background:  Concerns are arising about Acinetobacter baumannii (A. baumannii) gaining resistance to tigecycline (TGC). AdeB efflux pump gene has been strongly associated with TGC resistance and usnic acid (UA) has been postulated to act as an efflux pump inhibitor. This study aimed at determining the antibacterial effect of UA and its effect on TGC susceptibility and on adeB gene expression in TGC non susceptible multidrug resistant (MDR) A. baumannii clinical isolates. Methods: Fifty multidrug resistant Acinetobacter baumannii isolates were identified as TGC resistant using disc diffusion method (DD). Minimal inhibitory concentration (MIC) of TGC and UA were determined using microbroth dilution (MBD) test. Checkerboard synergy test was conducted on TGC non susceptible isolates to determine UA effect on MIC of TGC. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to measure adeB gene expression before treatment, after treatment with TGC alone and with TGC/UA combination. Results: Tigecycline non susceptibility determined by MIC values occurred in 44/50 (88%) isolates.  UA had antibacterial activity against 44/44(100%) of the isolates. Reduction in MIC of TGC in response to UA was detected in 10/44 (22.3 %) isolates in which high adeB gene expression was detected. In addition, 9/10 (90%) of them revealed highly significant reduction in adeB gene expression after treatment with TGC/UA combination. Conclusion: UA has an in vitro antibacterial action against TGC non susceptible A. baumannii with the potential ability of TGC and UA to synergistically act against them. Such synergism can be significantly attributed to UA efflux pump inhibitory action.