Sonchus erzincanicus (Asteraceae) is a plant species that is endemic to Turkey and grows in Erzincan. Lipid peroxidation inhibition, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, and total phenolic content of the methanol and water extracts of the plant were determined. Antimicrobial activity of the extracts was also tested against 107 clinical isolates of human pathogenic microorganism strains belonging to Enterobacter aerogenes, Escherichia coli, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus, and Candida albicans using the disc-diffusion method, and the minimal inhibition concentration (MIC) value of each active extract was determined. The extracts were observed to have antioxidant potential, although no better than that of BHT (butylated hydroxytoluene), which is a well-known commercial antioxidant. The extracts exhibited antimicrobial effects on all the tested microorganism species, except for P. aeruginosa and E. aerogenes. In addition, antifungal activity of the methanol extract against C. albicans was better than that of the water extract (inhibition zone: 25 mm; MIC: 0.156 mg/mL). As a consequence, it was observed that the methanol and water extracts of Sonchus erzincanicus had in vitro lipid peroxidation inhibition an I antimicrobial effects against some microorganisms.