This paper investigates how legal cannabis access affects student performance. Identification comes from an exceptional policy introduced in the city of Maastricht which discriminated legal access based on individuals' nationality. We apply a difference-in-difference approach using administrative panel data on over 54,000 course grades of local students enrolled at Maastricht University before and during the partial cannabis prohibition. We find that the academic performance of students who are no longer legally permitted to buy cannabis increases substantially. Grade improvements are driven by younger students, and the effects are stronger for women and low performers. In line with how THC consumption affects cognitive functioning, we find that performance gains are larger for courses that require more numerical/mathematical skills. We investigate the underlying channels using students' course evaluations and present suggestive evidence that performance gains are driven by improved understanding of material rather than changes in students' study effort. [This paper was produced as part of the Centre's Education Programme. The Centre for Economic Performance is financed by the Economic and Social Research Council.]