Despite significant public concerns voiced about wearing face masks and reports from healthcare workers of adverse effects on cognition, research into potential adverse effects remains limited. The present trial investigated the effects of wearing a surgical face mask for prolonged periods on cognitive functioning and mood. We tested 42 university students (18-36 years old) using a controlled counterbalanced crossover design that involved a mask session and a control session, separated by 1 week. The two sessions were identical except that on the day of the mask session, participants were asked to wear a surgical mask for at least 8 h and to continue wearing it while visiting our laboratory, during which cognitive performance and mood were assessed as per the control session. Results showed that participants reported feeling less happy and more tense during the mask session compared to no-mask control. Additionally, cognitive performance differed between the two sessions for a selective attention task, reflecting slower response latencies during the mask session, which for the most part appeared to be driven by those who felt anxious wearing the mask. Although significant differences emerged for only two of six mood scales and one of eight cognitive tests, the evidence of adverse effects in a university population signals a need for research investigating vulnerable populations. Individuals with elevated anxiety may be particularly important to target.Trail Registration: This randomized controlled trial was retrospectively registered (ACTRN12620001215910). Date registered: 16/11/2020, retrospectively registered.