BACKGROUND: Mobile health (mHealth) has been used to promote sexual and reproductive health (SRH) education and services; however, little is known about the use of mHealth to improve safe abortion knowledge and access to safe abortion services among female sex workers (FSWs). This study evaluated the feasibility and effectiveness of intervention through changes in knowledge on safe abortion and changes in perceived barriers to safe abortion services among FSWs in Vietnam. METHODS: mobile app was developed as an interactive platform to deliver safe abortion education and referral to safe abortion services through short messaging services (SMS) enhanced by tele-counseling for 512 FSWs in Hanoi, Vietnam. A pretest-posttest evaluation was conducted using questionnaire-based phone interviews administered to 251 participants at baseline and 3 months following the intervention. Non-parametric tests evaluated the change in abortion knowledge, behaviors, and perceived barriers to safe abortion. RESULTS: There were significant improvements in the knowledge on safe abortion among the study participants. Specifically, FSWs' knowledge of correct gestational ages (≤22 weeks) for medical abortion increased from 78.9% at baseline to 96.8% (P=0.001). Knowledge of correct gestational ages for medical abortion at the private clinic increased from 45.3% to 63.1% (P=0.001). Knowledge on the consequences of unsafe abortion increased from 75.2% to 92.1% (P=0.001). In addition, perceived stigma and discrimination when seeking safe abortion decreased from 36.5% to 27.8% (P=0.036) and worry about the lack of confidentiality decreased from 23.3% to 15.5% (P=0.035). CONCLUSIONS: The evaluation results showed the initial effectiveness of a mobile app-based intervention in improving access to safe abortion information and services among FSWs. A future study is needed to establish the efficacy of the intervention for scaling up in Vietnam and elsewhere.