Due to the COVID-19 pandemic and its extensive effects, the incidence of posttraumatic stress disorder (PTSD) symptoms is rapidly increasing in China. This research aimed to assess the efficacy and acceptability of a mobile application delivering Acceptance and Commitment Therapy (ACT) in reducing PTSD symptoms. 221 Chinese individuals with elevated PTSD symptoms were randomly assigned to app-delivered ACT (ACT condition), app-delivered mindfulness (MI condition), or a waitlist (WL condition). Assessments were performed pre- and post-intervention. The results showed that participants in both the ACT and MI groups had significantly greater improvements across mental health outcomes compared to the WL group. No significant differences were observed between the ACT and MI groups except for psychological flexibility, which improved more in ACT than MI (d = -0.37). Compared to WL, the ACT group showed a greater improvement in PTSD symptoms (d = -0.79), anxiety (d = -0.62), depression (d = -0.51), posttraumatic growth (d = 0.46), and psychological flexibility (d = 0.76). The drop-out rates in the ACT and MI were 25.76% and 39.71%, respectively. Participants in the ACT condition reported medium program satisfaction. The study suggests app-delivered ACT is efficacious in reducing PTSD symptoms and improving overall mental health among Chinese adults.