Accurate decision-making under imminent threat is critical to survival. However, arousal states, regulated by the neuromodulator norepinephrine, can cause both attentiveness and anxiety. Thus, adaptive decision making that scales with threat valence is crucial. To study proper decision-making under threat and the role of norepinephrine, we developed a novel task in which mice descend from a height toward an illusion of safe and unsafe visual cues. This virtual stimulus --- where there is a mismatch between actual safety and perceived safety --- relies on the evolutionarily conserved, innate fear of heights to drive an arousal state. Fiber photometry of the locus coeruleus (LC), the deep-brain noradrenergic nucleus, as well as the medial prefrontal cortex, a target of noradrenergic projections from LC, revealed increased arousal that tracks perceived safety. A generalized linear mixed model was applied to analyze task performance and behavioral variables, revealing that previous choices may inversely affect current decisions, aligning with reinforcement learning models. This task and the accompanying data highlight that the norepinephrine system is critical for decision-making under threat.