Due to the nature of the events in war, posttraumatic stress disorder (PTSD) is more difficult to treat in veterans than in other people. Nowadays, using art-making methods such as mandala coloring has attracted the attention of researchers. This randomized controlled trial was conducted with 84 male veterans diagnosed with PTSD. Veterans were recruited by convenience sampling method and randomly assigned to an intervention group (n = 42) and a control group (n = 42). Participants in the intervention group colored a mandala, whereas the control group colored freely for 15 minutes 2 times a week for 3 weeks. All participants completed Brown and Ryan's Mindful Attention Awareness Scale and Posttraumatic Stress Disorder Checklist for diagnostic and statistical manual of mental disorders, fifth edition (DSM-5). Mean baseline mindfulness scores were not significantly different between the mandala coloring group and the free coloring group (41.73 ± 5.29 vs 40.23 ± 4.59, P = .169). After the intervention, the mean mindfulness score was significantly higher in the mandala coloring group than in the free coloring group (52.23 ± 5.89 vs 46.04 ± 7.28, P < .001). Mean baseline PTSD scores were not significantly different between the mandala coloring group and the free coloring group (65.45 ± 5.89 vs 67.47 ± 4.66, P = .085). After the intervention, the mean PTSD score was significantly lower in the mandala coloring group than in the free coloring group (47.21 ± 5.77 vs 52.38 ± 5.52, P < .001). Both mandala coloring and free coloring increase mindfulness and decrease PTSD symptom severity. However, the mandala coloring method was more effective.