The Neoproterozoic contains several pronounced negative carbon isotope excursions that have been the topic of intense debate. The foremost of these, the “Shuram excursion,” represents the largest known carbon isotope excursion in Earth’s history. These negative carbon isotope excursions have been variably interpreted to record primary seawater values and massive carbon cycle perturbations, diagenetic alteration, or porewater authigenic carbonate formation. Although there are abundant examples of recent and Phanerozoic authigenic carbonates with markedly negative carbonate carbon isotope values, these carbonates are clearly identifiable as diagenetic products, making it difficult to link them to Neoproterozoic carbon isotope excursions. Here, we report the occurrence of a Middle Triassic, shallow-marine and lagoonal succession that contains a negative carbon isotope excursion in fine crystalline and peloidal carbonates that is comparable—in terms of its magnitude and stratigraphic structure and variability—to several Neoproterozoic carbon isotope excursions. A coupled petrographic and multiple isotope (C-O-Sr-U) approach suggests that the excursion was driven by carbonate precipitation within anoxic porewaters. Extensive carbon precipitation in the upper portion of the sediment pile was likely linked to inhibited bioturbation and a high background carbonate saturation state in an evaporative setting. The discovery of a Phanerozoic authigenic carbon isotope excursion bolsters the case that some stratigraphically continuous Neoproterozoic negative carbon isotope excursions may be tied to carbonate formation within the sediment pile.