The dramatic increase in antimicrobial resistance (AMR) in recent decades has created an urgent need to develop new antimicrobial agents and compounds that can modify and/or block bacterial resistance mechanisms. An understanding of these resistance mechanisms and how to overcome them would substantially assist in the development of new antibiotic chemotherapies. Bacteria may develop AMR through multiple differing mechanisms, including modification of the antibiotic target site, limitation of antibiotic uptake, active efflux of the antibiotic, and via direct modification and inactivation of the antibiotic. Of these, efflux pumps and the production of β-lactamases are the most common resistance mechanisms that render antibiotics inactive. The development of resistance-modifying agents (particularly those targeting efflux pumps and β-lactamase enzymes) is an important consideration to counteract the spread of AMR. This strategy may repurpose existing antibiotics by blocking bacterial resistance mechanisms, thereby increasing the efficacy of the antibiotic compounds. This review focuses on known phytochemicals that possess efflux pump inhibitory and/or β-lactamase inhibitory activities. The interaction of phytochemicals possessing efflux pumps and/or β-lactamase inhibitory activities in combination with clinical antibiotics is also discussed. Additionally, the challenges associated with further development of these phytochemicals as potentiating agents is discussed to highlight their therapeutic potential, and to guide future research.