Deep-time hyperthermal/transient warming events provide critical analogs for modern climatic warming and its impacts. However, all ancient hyperthermals documented to date have occurred under greenhouse climate states, limiting their relevance for understanding the trajectory of present-day climate change. Here, we present conodont oxygen isotope data and document a transient global warming event at ~304 Ma during the Late Paleozoic Ice Age, herein termed the Kasimovian-Gzhelian Thermal Maximum (KGTM). Carbon and mercury isotope records indicate large-scale carbon release and the development of photic-zone euxinia during the KGTM, coincident with a marine biocrisis. The KGTM was initiated by a modest carbon release and temperature increase associated with potentially enhanced volcanic activity at an orbital eccentricity maximum, which likely crossed a climatic tipping point and triggered massive carbon release and abrupt large-scale warming. These findings demonstrate that transient warming events and severe oceanic deterioration can be caused by a modest thermal perturbation within an icehouse climate state analogous to that of the modern Earth.