Biocompatible　nano-antioxidants　composed　of　natural　molecules/materials,　such　as　dopamine　and　melanin,　are　of　great　interest　for　diverse　biomedical　applications.　However,　the　lack　of　understanding　of　the　precise　structure　of　these　biomaterials　and　thus　the　actual　dose　of　effective　components　impedes　their　advancement　to　translation.　Herein,　a　strategy　to　mimic　in　situ　melanin　formation　and　explore　its　antioxidative　applications　is　reported,　by　developing　a　PEGylated,　phenylboronic-acid-protected　L-DOPA　precursor　(PAD)　that　can　self-assemble　into　well-defined　nanoparticles　(PADN).　Exposure　to　oxidative　species　leads　to　deprotection　of　phenylboronic　acids,　transforming　PADN　to　PEG-L-DOPA,　which,　similar　to　the　biosynthetic　pathway　of　melanin,　can　be　oxidized　and　polymerized　into　an　antioxidative　melanin-like　structure.　With　ultrahigh　stability　and　superior　antioxidative　activity,　the　PADN　shows　remarkable　efficacy　in　prevention　and　treatment　of　acute　liver　injury/failure.　Moreover,　the　in　situ　structure　transformation　enables　PADN　to　visualize　damaged　tissue　noninvasively　by　photoacoustic　imaging.　Overall,　a　bioinspired　antioxidant　with　precise　structure　and　site-specific　biological　activity　for　theranostics　of　oxidative　stress-related　diseases　is　described.