Introduction: Caffeine is the most widely consumed psychoactive substance across the world reported to improve cognitive performance. Hence, this study aims to assess the effect of caffeine on working memory and sustained attention of undergraduate medical students. Methods: A randomized controlled trial (RCT) study was conducted within 2 months in the University of Cyberjaya, Cyberjaya, Selangor. Sixty-three students volunteered to participate and were randomized into two groups; caffeinated and controlled. Subjects with known medical illnesses (e.g., peptic ulcer disease, severe anxiety, seizure, metabolic diseases, and heart diseases), sleep less than 6 hours, and last caffeine intake less than 12 hours were excluded from this study. The subjects completed two tests (pre- and post- intervention) each for working memory and sustained attention, i.e., Hopkins Verbal Learning Test and Serial Seven Subtraction Ttest respectively after filling a survey form on caffeine consumption. Results: Out of 32 students in the caffeinated group, only 37.5% (p = 0.719) showed improvements in working memory, whereas 71.9% (p = 0.721) showed improvements in sustained attention. Conclusion: The results from this study suggest that acute ingestion of low doses of caffeine is not statistically significant to improve working memory and sustained attention.