Objectives The present study investigated the effectiveness of an 8-week mindfulness mobile phone app on women’s depression, anxiety, stress and mindful attention/awareness in the postnatal period. Methods The study enrolled 99 mothers of a child under 1 year old, and randomly assigned them to intervention (n = 49, mean age = 31.11, SD = 4.30, years) and control (n = 50, mean age = 31.35, SD = 5.29, years) groups. Multiple regression examined intervention effects on depression, anxiety, stress and mindful attention/awareness measured post-intervention and at 4-week follow-up, controlling for the baseline and post-intervention measurement of the specific outcome, respectively. Results The intervention group showed significant decreases in depression, anxiety and stress levels and an increase of mindful attention/awareness post-intervention compared to the control group, with medium to large effect sizes after controlling for effects of corresponding variables at baseline. The intervention group showed further decrease in depression and stress levels and an increase in mindful attention/awareness at 4 weeks post-intervention compared to the control group, with small to medium effect sizes, after controlling for effects of corresponding variables at post-intervention. Conclusions The outcomes of the study suggest that delivery of mindfulness via smartphones could be a viable and affordable resource for reducing postnatal depression, anxiety and stress.