Aviation security strongly depends on screeners' performance in the detection of threat objects in x‐ray images of passenger bags. We examined for the first time the effects of stress and stress‐induced cortisol increases on detection performance of hidden weapons in an x‐ray baggage screening task. We randomly assigned 48 participants either to a stress or a nonstress group. The stress group was exposed to a standardized psychosocial stress test (TSST). Before and after stress/nonstress, participants had to detect threat objects in a computer‐based object recognition test (X‐ray ORT). We repeatedly measured salivary cortisol and X‐ray ORT performance before and after stress/nonstress. Cortisol increases in reaction to psychosocial stress induction but not to nonstress independently impaired x‐ray detection performance. Our results suggest that stress‐induced cortisol increases at peak reactivity impair x‐ray screening performance.