OBJECTIVES: Stress has been shown to have a number of negative effects on health over time. Mindfulness interventions have been shown to decrease perceived stress but access to interventions is limited. Therefore, the effectiveness of an online mindfulness course for perceived stress was investigated. DESIGN: A preliminary evaluation of an online mindfulness course. PARTICIPANTS: This sample consisted of 100 self-referrals to the online course. The average age of participants was 48 years and 74% were women. INTERVENTIONS: The online programme consisted of modules taken from Mindfulness Based Stress Reduction and Mindfulness Based Cognitive Therapy and lasted for approximately 6 weeks. PRIMARY AND SECONDARY OUTCOME MEASURES: Participants completed the Perceived Stress Scale (PSS) before the course, after the course and at 1-month follow-up. Completion of formal (eg, body scan, mindful movement) and informal (eg, mindful meal, noticing) mindfulness activities was self-reported each week. RESULTS: Participation in the online mindfulness course significantly reduced perceived stress upon completion and remained stable at follow-up. The pre-post effect size was equivalent to levels found in other class-based mindfulness programmes. Furthermore, people who had higher PSS scores before the course reported engaging in significantly more mindfulness practice, which was in turn associated with greater decreases in PSS. CONCLUSIONS: Because perceived stress significantly decreased with such limited exposure to mindfulness, there are implications for the accessibility of mindfulness therapies online. Future research needs to evaluate other health outcomes for which face-to-face mindfulness therapies have been shown to help, such as anxiety and depressive symptoms.