Pyrrolizidine alkaloids (PAs) are among the most common phytotoxins with emerging evidence to contaminate soil, water, and nearby plants. Humans are frequently exposed to PA-contaminated food products or PA-containing herbal remedies. The reactive metabolites of PAs rapidly alkylate DNA to form pyrrole-DNA adducts (PDAs) and seed a population of mutations that initiate tumorigenesis in experimental models. However, the clinical evidence of PA-induced initial DNA damage is absent. This study developed a liquid biopsy approach for detecting PDAs in blood to diagnose PA-induced genotoxicity. Blood DNA samples, extracted from PA-exposed rats, were reacted with silver nitrate to cleave the N-linkage between conjugated DNA bases and pyrrole. The released pyrrole was chemically derivatized to a stable product which was measured by liquid chromatography-tandem mass spectrometry. Using this method, PDAs were, for the first time, detected in blood samples from 18 patients with PA-induced liver injury. Notably, PDAs were detectable in patients blood samples collected one month after hospital admission, indicating adequate persistence of PDAs for use as a clinical biomarker of PA-induced genotoxicity. Our findings provide clinical indication of PA-induced genotoxicity, which warrants early detection of PDAs as an actionable approach for prevention of PA-associated carcinogenesis.