Peroxynitrite (ONOO−) has emerged as a critical biomarker for detecting pathological alterations in biological stroma. Notably, it has strong correlation with the pathogenesis of conditions like non-alcoholic fatty liver disease (NAFLD), underlining its utility in both identifying and elucidating such diseases. In this work, we report a novel fluorescent probe, TEM-NS, innovatively crafted using an enhanced dicyanoisophorone scaffold, aimed at the precise quantification of ONOO−. Characterized by its turn-on fluorescence response, TEM-NS exhibits an emission profile and a pronounced Stokes shift superior to those of traditional dicyanoisophorone derivatives. Its exceptional specificity and photostability, combined with superior sensitivity in a biological milieu, are noteworthy. Importantly, application of TEM-NS within a NAFLD model substantiated its capability to detect elevated ONOO− levels in liver tissue, thereby affirming its applicability for in-depth analysis of oxidative stress-related disorders.