Stress, anxiety and depression are relatively common problems among university students. This study examined whether an online psychological intervention aiming at enhancing the wellbeing of university students could be an effective and practical alternative for meeting the needs of a university population. University students (N = 68; 85% female; 19–32 years old) were randomly assigned to either a guided seven-week online Acceptance and Commitment Therapy (iACT) intervention or a waiting list control condition (WLC). A between-groups pre–post (iACT vs WLC) design with 12-month follow-up for the iACT participants was conducted. The intervention participants were offered two face-to-face meetings, completed online exercises during a five-week period, and received personal weekly written feedback via the website from their randomly assigned, trained student coaches. Waitlist participants were offered the intervention program soon after the post measurements. Results in this small efficacy trial showed that the iACT participants had significantly higher gains in wellbeing (between group, d = 0.46), life satisfaction (d = 0.65), and mindfulness skills (d = 0.49). In addition, iACT participants' self-reported stress (d = 0.54) and symptoms of depression (d = 0.69) were significantly reduced compared to the participants in the control group. These benefits were maintained over a 12-month follow-up period (within iACT group, d = 0.65–0.69, for primary measures). The results suggest that an online-based, coach-guided ACT program with blended face-to-face and online sessions could be an effective and well-accepted alternative for enhancing the wellbeing of university students.