The peptidomimetic H-[NLys-tBuAla]6-NH2 (CEP-136), which exhibits low inherent antimicrobial activity against Gram-negative bacteria (MIC = 16-64 μM), was shown to significantly potentiate the antibacterial activity of several clinically important antibiotics against the human pathogens Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. Thus, the antibacterial spectrum of rifampicin, clarithromycin, and azithromycin could be extended to include also these Gram-negative bacteria. Additionally, the potentiation effect was demonstrated in a panel of clinically relevant multidrug-resistant isolates including extended-spectrum β-lactamase (ESBL)- and carbapenemase-producing as well as colistin-resistant strains. For some peptidomimetic-antibiotic combinations, the strong synergy corresponded to a more than 50-fold reduction of the minimal inhibitory concentration of the antibiotic. Mechanistic studies indicate that the potentiation arises from a permeabilization effect exerted on the outer membrane lipopolysaccharide layer of the Gram-negative bacteria without significant disruption of the inner membrane. Furthermore, the peptidomimetic enhancer exhibited only a marginal effect on the viability of mammalian HepG2 cells even at concentrations 100-fold higher than that enabling the antibiotic enhancement. Also, a low hemolytic activity combined with limited in vivo acute toxicity of CEP-136 in healthy mice allowed in vivo validation of the potentiation effect on both rifampicin and azithromycin treatment in a murine peritonitis model. Thus, CEP-136 is an interesting hit compound for further development of effective adjuvants for repurposing antibiotics for use against infections by multidrug-resistant Gram-negative bacteria.