Bioluminescence imaging (BLI) is a powerful, noninvasive imaging method for animal studies. NanoLuc luciferase and its derivatives are attractive bioluminescent reporters recognized for their efficient photon production and ATP independence. However, utilizing them for animal imaging poses notable challenges. Low substrate solubility has been a prominent problem, limiting brightness, while the susceptibility of luciferins to auto-oxidation by molecular oxygen in air increases handling complexity and poses an obstacle to obtaining consistent results. To address these issues, we developed a range of caged PEGylated luciferins with increased auto-oxidation resistance and water solubility of up to 25 mM, resulting in substantial bioluminescence increases in mouse models. This advancement has created the brightest and most sensitive luciferase-luciferin combination, enabling high-speed video-rate imaging of freely moving mice with brain-expressed luciferase. These innovative substrates offer new possibilities for investigating a wide range of biological processes and are poised to become invaluable resources for chemical, biological, and biomedical fields.