Arousal—our internal state of alertness—shapes attentional performance, yet its interaction with spatial attention is not well understood. This study examined how tonic arousal, indexed by pre-stimulus pupil diameter, modulates attentional engagement and reorienting in a Posner cueing task. Thirty-one participants completed the task while pupil size was continuously recorded. Cue validity distinguished low-demand (valid) from high-demand (invalid) trials, and reaction times were analyzed relative to arousal fluctuations. Results showed an inverted U-shaped relationship between arousal and performance, but only in invalid trials requiring attentional reorienting, suggesting that moderate arousal enhances cognitive flexibility in line with the Yerkes–Dodson law and adaptive gain theory. Valid trials showed little arousal sensitivity, indicating routine engagement operates independently of arousal. The overall cueing effect remained unaffected, likely due to opposing influences and a validity-biased design. These findings indicate that arousal selectively facilitates attentional reorienting under conflict, potentially via locus coeruleus–norepinephrine and cholinergic mechanisms, with implications for optimizing performance in high-stakes settings and interventions for conditions such as ADHD and PTSD.