Background: Primary dysmenorrhea is common among women of reproductive age. Nonsteroidal anti-inflammatory drugs and oral contraceptives are effective treatments, although the failure rate is around 20% to 25%. Therefore additional evidence-based treatments are needed. In recent years, the use of smartphone applications (apps) has increased rapidly and may support individuals in self-management strategies. Objective: We aimed to investigate the effectiveness of app-based self-acupressure in women with menstrual pain. Materials and Methods: A 2-armed, randomized, pragmatic trial was conducted from December 2012 to April 2015 with recruitment until August 2014 in Berlin, Germany, among women aged 18 to 34 years with self-reported cramping pain of 6 or more on a numeric rating scale (NRS) for the worst pain intensity during the previous menstruation. After randomization, women performed either app- based self-acupressure (n = 111) or followed usual care only (n = 110) for 6 consecutive menstruation cycles. The primary outcome was the mean pain intensity (NRS 0−10) on the days with pain during the third menstruation. Secondary outcomes included worst pain intensity during menstruation, duration of pain, 50% responder rates (reduction of mean pain by at least 50%), medication intake, sick leave days, and body efficacy expectation assessed at the first, second, third, and sixth menstruation cycles. Results: We included 221 women (mean age, 24.0 years; standard deviation [SD], 3.6 years). The mean pain intensity difference during the third menstruation was statistically significant in favor of acupressure (acupressure: 4.4; 95% confidence interval [CI], 4.0−4.7; usual care 5.0; 95% CI, 4.6−5.3; mean difference −0.6; 95% CI, − 1.2 to −0.1; P = .026). At the sixth cycle, the mean difference between the groups (−1.4; 95% CI, −2.0 to −0.8; P < .001) reached clinical relevance. At the third and sixth menstruation cycles, responder rates were 37% and 58%, respectively, in the acupressure group, in contrast to 23% and 24% in the usual care group. Moreover, the worst pain intensity (group difference −0.6; 95% CI, −1.2 to −0.02; and −1.4; 95% CI, −2.0 to −0.7), the number of days with pain (−0.4; 95% CI, −0.9 to −0.01; and −1.2; 95% CI, −1.6 to −0.7) and the proportion of women with pain medication at the third and sixth menstruation cycles (odds ratio [OR], 0.5; 95% CI, 0.3−0.9] and 0.3 (95% CI, 0.2−0.5) were lower in the acupressure group. At the third cycle, hormonal contraceptive use was more common in the usual care group than in the acupressure group (OR, 0.5; 95% CI, 0.3−0.97) but not statistically significantly different at the sixth cycle (OR, 0.6; 95% CI, 0.3−1.1]). The number of sick leave days and body efficacy expectation (self-efficacy scale) did not differ between groups. On a scale of 0 to 6, mean satisfaction with the intervention at the third cycle was 3.7 (SD 1.3), recommendation of the intervention to others 4.3 (1.5), appropriateness of acupressure for menstrual pain 3.9 (1.4), and application of acupressure for other pain 4.3 (1.5). The intervention was safe, and after the sixth cycle, two-thirds of the women (67.6%) still applied acupressure on all days with pain. Conclusion: Smartphone app−delivered self-acupressure resulted in a reduction of menstrual pain compared to usual care only. Effects were increasing over time, and adherence was good. Future trials should include comparisons with other active treatment options.