Stressful events trigger a complex physiological reaction – the fight-or-flight response – that can hamper flexible decision-making. Inspired by key neural and peripheral characteristics of the fight-or-flight response, here we ask whether acute stress changes how humans learn about costs and benefits. Participants were randomly exposed to an acute stress or no-stress control condition after which they completed a cost-benefit reinforcement learning task. Acute stress improved learning to maximize benefits (monetary rewards) relative to minimising energy expenditure (grip force). Using computational modelling, we demonstrate that costs and benefits can exert asymmetric effects on decisions when prediction errors that convey information about the reward value and cost of actions receive inappropriate importance; a process associated with distinct alterations in pupil size fluctuations. These results provide new insights into learning strategies under acute stress – which, depending on the context, may be maladaptive or beneficial - and candidate neuromodulatory mechanisms that could underlie such behaviour.