Antimicrobial resistance poses a public health threat, necessitating urgent research on phenolic compounds which enhance antimicrobial efficacy and regulate bacterial gene regulation. The study aimed to investigate the antibacterial activity of polyphenols from the by-products of the Portuguese red grape variety Cabernet Franc against antibiotic-resistant bacteria associated with foodborne pathogens (Escherichia coli and Listeria monocytogenes). Polyphenols are obtained from grape skins, seeds, and stems. The polyphenol profile of the extracts was determined using high-performance liquid chromatography (HPLC). Antimicrobial susceptibility testing was performed on eight foodborne bacteria from livestock and food-products, four E. coli and four L. monocytogenes, using the Kirby–Bauer disk diffusion method. The initial extract of 100 mg/mL was diluted with dimethyl sulfoxide (DMSO) to 75, 50, 25, and 10 mg/mL, and the results were expressed as minimum inhibitory concentrations (MICs). The study reveals that the by-product extracts of Cabernet Franc red grape contains various polyphenols, including phenolic acids, flavan-3-ols, flavonols, stilbenes, and anthocyanins. The total polyphenol content is directly proportional to the inhibition zone diameter, suggesting that they play a role in antibacterial activity. The seed extract demonstrated the highest antibacterial activity, as evidenced by its inhibitory effect (MIC 25 mg/ml) against three of eight bacterial strains tested, particularly against L. monocytogenes. This study shows that high concentrations of catechin and epicatechin in seeds have antibacterial effects, including antibacterial effects against multidrug-resistant strains. Catechins have bactericidal effects by producing hydrogen peroxide, which damages bacterial cell membranes but is less effective against Gram-negative bacteria. The study reveals that polyphenols, including catechins, have antibacterial effects against food-borne pathogens. The sensitivity of bacteria to polyphenols varies based on the type and structure of the polyphenols. Polyphenols are promising therapeutic agents, increasing antibiotic effectiveness in fighting foodborne bacteria. Their activity also suggests their potential as effective food preservatives.