The present study sought to examine the underpinnings of impaired strategic decision-making under stress. In contrast to previous laboratory-based research, we conducted a quasi-experiment in a real life stress situation. Specifically, we used the beauty contest game and compared the performance of a group of participants who were exposed to a real-life stressor (waiting to attend an exam at a university class) with a control group of participants who were not exposed to stress (waiting to attend a regular lecture at a university class). Furthermore, about half of the participants were instructed to write down what they believed another participant had assumed the average number in the beauty contest game to be and which target number she (or he) had chosen accordingly. The results showed that stress impaired strategic reasoning in the beauty contest game. Importantly, even when only including participants who understood the rules of the game in the analyses, stress still increased the numbers chosen in the beauty contest. Furthermore, we found that participants in the stress condition were significantly less likely to base their chosen number on their belief about other players’ choices. Hence, stress not only impairs understanding of the math behind the beauty contest game but also the degree of strategizing per se.