Plastic ingestion is a well-established threat to wildlife, inducing a series of lethal and sublethal physiological consequences. Yet monitoring the ongoing sublethal implications of plastic exposure in live wild populations remains challenging, making the development of a minimally invasive biomarker for plastic exposure a longstanding priority. In this study, we investigate the application of proteomic signatures identified in de Jersey, et al. (1) as a biomarker approach and validate across Procellariform seabird species of varying levels of plastic ingestion. Utilising data-independent mass spectrometry on plasma, we developed a consistent and reliably detectable biomarker signature with 96% accuracy for plastic ingestion from as little as 0.5 g of ingested plastic. Rather than relying on a single protein prone to biological and technical variability, our pathway-level approach integrates functionally related proteins, capturing consistent and conserved responses to plastic ingestion across species. These include elevated evidence of cell lysis, compromised stomach permeability and fibrosis, and a decrease in secreted proteins. The detected physiological patterns are aligned with emerging models of plastic-induced pathology and diseases, such as plasticosis. Our findings establish a foundation for a non-lethal, scalable tool capable of diagnosing plastic exposure across seabird taxa, with potential applications in broader ecological monitoring and conservation programs.