The objectives of this study were to obtain and characterise polyphenolic extracts from red grape pomace of Vitis vinifera L. cv Graciano via conventional solvent extraction (SE) and green supercritical fluid extraction (SFE) and to evaluate their in vitro antibacterial activity against susceptible and multidrug-resistant Escherichia coli strains of intestinal origin. The SE and SFE methods were optimised, and ultra-performance liquid chromatography/mass spectrometry (UPLC/QqQ-MS/MS) analysis revealed 38 phenolic compounds in the SE sample, with anthocyanins being the predominant polyphenols, and 21 phenolic compounds in the SFE samples, among which hydroxybenzoic acids and flavonols were the predominant compounds. The SE and SFE samples showed antibacterial activity against both antibiotic-susceptible and -resistant E. coli strains, and minimal inhibitory concentration values were in the range of 1–4 mg/mL. The activity was bacteriostatic in all cases, and it was shown that a higher content of total polyphenols correlated with a higher antibacterial activity of the extracts. This study shows that red grape pomace of Vitis vinifera L. cv. Graciano is a rich source of bioactive phenolic compounds that can become an important feedstock for additives and other upgraded products of biotechnological interest, which can help to modulate intestinal microbiota and combat bacterial antibiotic resistance.