Sleep difficulties are widespread among international students. Internet-based interventions are suggested as a low-threshold treatment option but may require cultural adaptation among culturally diverse populations. The present pilot study investigated the effectiveness and acceptance of an internet-based intervention to improve sleep difficulties in international students. A total of 81 international students of 36 nationalities were randomly assigned to the intervention (n = 41) or waitlist control group (n = 40). The intervention group received immediate access to a culturally non-adapted unguided internet-based sleep intervention consisting of three modules based on sleep hygiene and cognitive techniques to reduce rumination. At baseline, 4 and 12 weeks after randomisation, insomnia severity, measured by the Insomnia Severity Index, and secondary outcomes (sleep quality, depression, anxiety, perceived stress, well-being, presenteeism, mental health literacy) were assessed. Data were analysed using linear multi-level analyses. Additionally, satisfaction and perceived cultural appropriateness of the intervention were evaluated by international students after 4 weeks, and compared with ratings of German students, who represent the original target group. Insomnia severity improved over time in the intervention group compared to the control group, revealing a significant estimated mean difference of −5.60 (Hedges’ g = 0.84, p < 0.001) after 12 weeks. Satisfaction and perceived cultural appropriateness was high and comparable to that of German students. The present study shows that a culturally non-adapted internet-based sleep intervention can be a low-threshold treatment option to help meet the high demand for mental healthcare among international students. It thus indicates that cultural adaptation might not represent a precondition for providing effective internet-based sleep interventions to this target group.