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<i>Background and Objectives:</i> Limited research has evaluated the effects of acute exercise on cognition under different conditions of inspired oxygenation. Thus, the purpose of this study was to examine the effects of high-intensity interval exercise (HIE) under normoxia (inspired fraction of oxygen (FIO₂): 0.209) and moderate hypoxia (FIO₂: 0.154) on cognitive function. <i>Design:</i> A single-blinded cross-over design was used to observe the main effects of exercise and oxygen level, and interaction effects on cognitive task performance. <i>Methods:</i> Twenty inactive adults (10 males and 10 females, 19&#8315;27 years old) performed a cognitive task (i.e., the Go/No-Go task) before and immediately after an acute bout of HIE under normoxic and hypoxic conditions. The HIE comprised 10 repetitions of 6 s high-intensity cycling against 7.5% body weight interspersed with 30 s passive recovery. Heart rate, peripheral oxygen saturation (SpO₂) and rating of perceived exertion were monitored. <i>Results:</i> The acute bout of HIE did not affect the reaction time (<i>p</i> = 0.204, &#951;² = 0.083) but the accuracy rate decreased significantly after HIE under both normoxic and hypoxic conditions (<i>p</i> = 0.001, &#951;² = 0.467). Moreover, moderate hypoxia had no influence either on reaction time (<i>p</i> = 0.782, &#951;² = 0.004) or response accuracy (<i>p</i> = 0.972, &#951;² &lt; 0.001). <i>Conclusions:</i> These results indicate that an acute session of HIE may impair response accuracy immediately post-HIE, without sacrificing reaction time. Meanwhile moderate hypoxia was found to have no adverse effect on cognitive function in inactive young adults, at least in the present study.