The number of university students experiencing mental disturbances is increasing. Due to the impact it has on students’ academic life, there is a need for an efficient, convenient, and cost- effective intervention which can be widely applied to university populations. Acceptance and Commitment Therapy (ACT) is an effective behavioural treatment for a range of psychological disorders. ACT has a transdiagnostic feature, as it focus on educating individuals to be aware of their internal experiences, such as thoughts and feelings and to accept them, instead of suppressing or avoiding them. The fundamental goal of ACT is to promote individuals’ psychological flexibility when faced with challenging events by addressing their valued life goals. ACT CompanionTM app is an ACT-based smartphone app which incorporates exercises to assist users achieve their treatment goals. I used a single- subject design to examine the efficacy of the ACT CompanionTM app in reducing measures of stress, anxiety, depression, and psychological flexibility for 9 university participants. Graphs showed that app use lowered daily stress levels for all participants. At a group level, there was a significant decrease in anxiety, stress, depressive symptoms, and negative emotions. However, no significant effect of app use on mindfulness and psychological flexibility. Overall, the results suggest that the ACT CompanionTM app is a promising approach for improving mental wellbeing among university students.